Email signing & encryption
Prove a message came from you — and encrypt it so only they can read it
Email was designed with no way to prove who sent a message. An S/MIME certificate adds the proof — a signature your recipient can check, and encryption only they can undo.
- Nobody can send mail as youThe fake will not carry your signature, and your recipient will see that it does not.
- Only the recipient can read itEncrypted end to end. Not your mail provider, not anyone with the server backups.
- Log in without a passwordThe same certificate authenticates you to VPNs, intranets and admin panels.
- Working this afternoonClick a link in your inbox and it is issued. Recipients install nothing.
Issued by publicly trusted certificate authorities — your recipient’s mail client verifies the signature with nothing installed
- We do not issue it — they doThe certificate authority signs your certificate, not us. Identical file, identical trust in every mail client, lower price.
- Lowest price guaranteeFind the same certificate cheaper anywhere else and we match it. Tell us where you saw it.
- We tell you before you buyEmail signing certificates cannot be refunded once issued, so we would rather talk you out of the wrong one first. Ask us and we will say which level you actually need. Read the policy.
- Free set-up help from engineersCollecting it, exporting the key backup, and turning signing on in Outlook, Apple Mail or Thunderbird.
The reason this is not optional any more
Every mail client on earth shows you a friendly name and an address in the From line, and neither is verified. Typing them is all it takes. The scam that follows is not clever and does not need to be: someone watches a mailbox long enough to learn who signs off payments, then sends one message, at the right moment, asking for a bank detail to be updated. Nothing in the message looks wrong, because nothing about it is wrong — it is a genuine email that simply is not from who it claims.
Re: October remittance — updated bank details
Hi — before you run tomorrow’s payment run, our account has moved to the new facility. Details attached. Can you get this in before 4pm?
No signature · sender unverifiedBoard pack for Thursday
Papers attached, and the revised forecast is in the second tab. Shout if anything looks off before I circulate it wider.
Signed by d.osei@acmetrading.comTwo messages, one afternoon, from the same name. The difference is one character in the domain and one thing the reader cannot see: only the second carries a signature the mail client could check. With signing turned on across the team, the first one stands out on its own.
- Your domain protections do not travel with the messageSPF, DKIM and DMARC check the server that handed the mail over. They are invisible to the person reading it, they say nothing about which human wrote it, they do not survive most forwarding, and they do nothing at all when the attacker registers a lookalike domain of their own — which is what they do.
- The finance team cannot tell, and it is not their faultTraining people to spot a message with no visible tell is not a control. A signature is: it is either present and valid, or it is not, and the mail client makes that judgement rather than the reader.
- The confidential half of the problem is quieterOrdinary mail sits in readable form on every server it crosses, in every backup taken of them, and in the recipient’s mailbox for years. TLS protects the hop, not the message. One mailbox breach later, all of it is readable at once.
- Afterwards, you have to prove what you sentDisputes over what was agreed, and when, are settled by evidence. An unsigned email is an assertion. A signed one carries cryptographic proof of author and of the fact that not one character has been altered since.
What it costs to be wrong once, against what it costs not to be
The FBI’s Internet Crime Complaint Centre logged 21,489 business email compromise reports in 2023, with adjusted losses of $2.9 billion in the United States alone — an average of roughly $135,000 for every incident reported.
Figures: FBI IC3 Internet Crime Report 2023, and the IC3 public service announcement of 11 September 2024, which put worldwide exposed losses at $55.49 billion between October 2013 and December 2023. The comparison bar is drawn to scale.
Three guarantees, one file
A certificate is not a security product you have to operate. It is a small file you install once, after which your mail client does all three of these on every message without being asked again.
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Authenticity
The signature can only have been produced by the private key that lives on your device, and that key never leaves it. A recipient’s mail client checks the signature against the certificate authority’s root and shows a result. There is nothing for them to judge and nothing for them to install.
Answers: did this really come from you? -
Integrity
The signature covers the message and its attachments. Change a digit in an account number, a figure in a spreadsheet or a word in a contract anywhere between you and the reader, and the signature stops verifying. There is no way to alter signed mail quietly.
Answers: is this exactly what you sent? -
Confidentiality
Once someone holds your certificate they can reply encrypted, and only your private key opens the result. Not the mail provider, not the server administrator, not whoever eventually gets hold of the backups. This is the part that satisfies most written confidentiality obligations.
Answers: who else could ever read it?
What actually happens when you press send
Worth two minutes, because it explains both why the recipient needs nothing installed and why losing your key is the one mistake that cannot be undone.
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1
Your key signs it
Your mail client takes a fingerprint of the message and encrypts that fingerprint with the private key held on your device. That is the signature. The message itself is unchanged.
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2
The certificate travels with it
Your certificate goes out attached to the message. It contains your public key and the address, name or company the authority checked before issuing it.
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3
Their client checks it
The public key undoes the fingerprint, and it is compared with a fresh fingerprint of the message as received. Match, and nothing was altered. Then the certificate is traced back to its root.
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4
They can reply encrypted
Having received one signed message, their client now holds your public key. Anything they encrypt with it can be opened by your private key and by nothing else.
This is why S/MIME works with everyone rather than only with people who also bought something: the recipient needs no certificate, no account, no plug-in and no instructions. Every mainstream mail client — Outlook, Apple Mail, Thunderbird, iOS and Android — already trusts these certificate authorities and does the checking silently.
Do you actually need one? Here is the honest test
We would rather you bought nothing than bought the wrong thing. If none of the following is true of you, an email certificate is a nicety and you can close this page. If two or more are true, it is the cheapest control on your risk register.
- Somebody in your organisation can change bank details or release a payment on the strength of an email
- You send personal, financial, medical or legal data to people outside your own network
- A regulator, insurer or customer contract requires encryption of data in transit — GDPR Article 32, HIPAA, or an ISO 27001 statement of applicability
- You have already had, or narrowly avoided, an invoice-redirection or impersonation attempt
- Your signature on a message needs to mean something afterwards — instructions, approvals, filings, professional advice
- You want staff to reach an intranet, VPN or admin panel without another password to phish
- Your name or brand is worth impersonating, and a lookalike domain is a plausible attack
- You are asked to prove a message was authorised by the company, not merely sent from it
Where to start if you are unsure: put a mailbox-validated certificate on the three people who authorise money — usually the finance mailbox, whoever signs off invoices, and the director whose name gets impersonated. At our cheapest certificate that is $35.85 a year for all three. It changes nothing about how anybody works, and it removes the exact gap the invoice scam walks through. Widen it later if it earns its place.
Three levels — and the only thing that changes is what goes inside
Every certificate on this page uses the same cryptography and signs to the same standard. The price difference buys one thing: how much the certificate authority verifies before it writes your details into the file, and therefore how much a recipient can read out of it. The panels below show the actual subject line of the certificate at each level.
That you control the address
The authority emails a link to the address and you click it. Your address goes into the certificate; your name does not. That is enough to stop anyone signing as you, and enough to encrypt.
Inside the certificate
- E
- d.osei@acmetrading.com
- CN
- not verified at this level
- O
- not verified at this level
Right for: Individuals, small teams, and anyone who wants signed and encrypted mail working today without paperwork.
That you are who you say you are
You prove your legal identity with government photo ID, and your verified full name is written into the certificate beside the address. A recipient sees a person they can check, not just a mailbox.
Inside the certificate
- E
- d.osei@acmetrading.com
- CN
- David Osei
- O
- not verified at this level
Right for: Professionals signing on their own authority — lawyers, accountants, consultants, doctors — and anyone whose counterparty reads the name on a signature.
That you act for your company
Your identity is checked, and so is the company: that it legally exists, that its address is real, and that you are authorised to act for it. The registered company name goes into the certificate with your own.
Inside the certificate
- E
- d.osei@acmetrading.com
- CN
- David Osei
- O
- Acme Trading Limited
Right for: Anyone signing on the company’s behalf — contracts, tenders, purchase orders, regulatory filings — and organisations rolling certificates out to staff.
Every email certificate we sell, by what it proves
Inside each level the certificates are interchangeable in how they verify — a recipient cannot tell one authority from another in the result they see. What differs is the name on the invoice and the price, so buy on whichever of those matters to you.
That you control the address
Individuals, small teams, and anyone who wants signed and encrypted mail working today without paperwork.
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CPAC Basic - Personal Authentication Signature
The entry point to the CPAC range, and the one we sell most of. Mailbox validated, issued in minutes, and the cheapest certificate that keeps the upgrade path to Pro and Enterprise open.
$16$39save 59% for 1 year$29$69save 58% for 2 years · $14.50/yr -
Certum Basic ID Certificate
The cheapest publicly trusted S/MIME certificate we sell. Verifies in every mainstream mail client exactly as the others do; what you give up is brand recognition, not trust.
$11.95$49save 76% for 1 year$22.95$89save 74% for 2 years · $11.48/yr -
Sectigo Secure Email Certificate
The Sectigo-branded secure email certificate, off the same infrastructure as the Comodo one. Buy it when a policy document or purchase order names Sectigo.
$24.99$39save 36% for 1 year$37.99$69save 45% for 2 years · $19/yr -
Comodo Secure Email Certificate
The general-purpose Comodo email certificate: signing, encryption and client authentication in one, under the name most people search for.
$24$39save 38% for 1 year$44$69save 36% for 2 years · $22/yr -
Comodo Email Digital Signature
Comodo’s email signing certificate, aimed at people whose priority is a visible, verifiable signature on outbound mail.
$24.99 for 1 year$37.99 for 2 years · $19/yr -
Digital Signature Certificate for Email
Sectigo email signing certificate — the product to quote when a compliance list names Sectigo rather than Comodo.
$24.99$29save 14% for 1 year$42.99$49save 12% for 2 years · $21.50/yr -
Personal Digital Signature Certificate
A Comodo mailbox certificate sold as a straight personal signing product: same validation, bundled a little differently.
$19.99$39save 49% for 1 year$32.99$69save 52% for 2 years · $16.50/yr -
Certum Email ID - Individual
Certum’s individual mailbox certificate, a small step up from Basic ID in what is bundled with it.
$15$49save 69% for 1 year$28$89save 69% for 2 years · $14/yr
That you are who you say you are
Professionals signing on their own authority — lawyers, accountants, consultants, doctors — and anyone whose counterparty reads the name on a signature.
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CPAC Pro - Sectigo Personal Authentication Pro Certificate
Adds a checked government-ID identity, so your verified full name appears in the certificate rather than only your address. The usual choice for professional sign-off.
$49$59save 17% for 1 year$79$120save 34% for 2 years · $39.50/yr -
Certum Professional ID (S/MIME)
Certum’s identity-validated certificate: photo ID checked, your verified name inside, at close to half what the equivalents cost.
$29.95$99save 70% for 1 year$49.95$179save 72% for 2 years · $24.98/yr
That you act for your company
Anyone signing on the company’s behalf — contracts, tenders, purchase orders, regulatory filings — and organisations rolling certificates out to staff.
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CPAC Enterprise - Sectigo CPAC Enterprise Certificate
Full organisation validation on the Comodo range: your registered company name goes in alongside your own. The one to buy for contracts and tenders.
$69$79save 13% for 1 year$119$160save 26% for 2 years · $59.50/yr -
Certum Business ID
Organisation validated at a price closer to most rivals’ individual tier — the value pick for company signing, and the sensible base for a rollout.
$30.95$99save 69% for 1 year$49.95$179save 72% for 2 years · $24.98/yr -
Certum Enterprise ID
Certum’s full organisation-validated certificate, aimed at deployments across a whole company.
$36$119save 70% for 1 year$64$199save 68% for 2 years · $32/yr
Compare all 13 certificates side by side Both terms at once, every authority, with the assurance level against each
| Certificate | Brand | 1 year | 2 years | |
|---|---|---|---|---|
| Mailbox validated — That you control the address | ||||
| CPAC Basic - Personal Authentication Signature | Comodo CPAC | Details → | ||
| Certum Basic ID Certificate | Certum | Details → | ||
| Sectigo Secure Email Certificate | Sectigo | Details → | ||
| Comodo Secure Email Certificate | Comodo | Details → | ||
| Comodo Email Digital Signature | Comodo | Details → | ||
| Digital Signature Certificate for Email | Sectigo | Details → | ||
| Personal Digital Signature Certificate | Comodo CPAC | Details → | ||
| Certum Email ID - Individual | Certum | Details → | ||
| Individual validated — That you are who you say you are | ||||
| CPAC Pro - Sectigo Personal Authentication Pro Certificate | Comodo CPAC | Details → | ||
| Certum Professional ID (S/MIME) | Certum | Details → | ||
| Organisation validated — That you act for your company | ||||
| CPAC Enterprise - Sectigo CPAC Enterprise Certificate | Comodo CPAC | Details → | ||
| Certum Business ID | Certum | Details → | ||
| Certum Enterprise ID | Certum | Details → | ||
This is not SPF, DKIM or DMARC — and it is not a disclaimer
Almost everyone arriving here already has the domain-level records in place and has been told that means their email is protected. It does not mean that, and the difference is the reason invoice fraud still works against organisations with a perfect DMARC report.
| SPF, DKIM and DMARC | S/MIME certificate | |
|---|---|---|
| What is being verified | The server that handed over the message, and the domain it claimed. | The individual person who wrote it, and every byte of what they wrote. |
| Who sees the result | The receiving mail server. Nothing is shown to the reader. | The reader, in their own mail client, on every message. |
| Survives forwarding | Often not — forwarding commonly breaks the check. | Yes. The signature is part of the message and travels with it. |
| Stops a lookalike domain | No. The attacker’s own domain passes its own records perfectly. | Yes. They cannot produce your signature from any domain. |
| Encrypts the message itself | No. TLS protects a hop; the message rests in readable form at both ends. | Yes, end to end, readable only by the holder of the private key. |
| Proof afterwards | A server log, if anyone kept one. | Cryptographic proof of author and content, in the message itself. |
| What it is for | Stopping bulk spoofing of your domain and protecting deliverability. | Proving a particular human sent a particular message, and hiding it from everyone else. |
If what you actually need is a PDF signature Adobe trusts
This is the mistake that costs people money, so it is worth two minutes. Every certificate above can sign a PDF, and the signature is cryptographically real. But Adobe Reader will still show “At least one signature has problems”, because Adobe trusts its own list of certificate authorities — the Adobe Approved Trust List — and personal S/MIME certificates are not on it. If a client, a court or a regulator has to open your document and see a clean green tick with nothing configured at their end, you need a document signing certificate instead.
- On the Adobe Approved Trust List — verifies cleanly in Reader and Acrobat with no set-up at the other end
- Long-term validation, so the signature still verifies years after the certificate itself has expired
- Supplied on a hardware token or in an HSM, as the current requirements demand
- Signs Microsoft Office documents to the same standard
Getting it working
The certificate arrives as a file you install once. The step people get stuck on is not installation — it is realising that the private key is the certificate, and that it was generated on the machine you ordered from.
Order and validate
A validation link arrives at the address the certificate will carry. Mailbox-validated certificates are issued the moment you click it; the higher levels add a document check.
Minutes to 3 daysCollect it in the same browser
Collect the certificate in the browser you started the order in, because that is where the private key was generated. Collect it somewhere else and there is no key waiting to pair with it.
5 minutesExport a PKCS#12 backup
Export it as a .pfx or .p12 file with the private key, set a strong password, and store it somewhere durable. This one file is what you install on the laptop, the phone and the mail client.
5 minutesTurn signing on by default
Outlook, Apple Mail and Thunderbird each have a security setting where you choose the certificate and tick sign-every-message. Doing it by default is what makes an unsigned message from you look wrong.
10 minutes
Read this before you order
Two years is the longest anyone can legitimately sell you
Under the CA/Browser Forum’s S/MIME Baseline Requirements, in force since 1 September 2023, publicly trusted S/MIME certificates are capped at 825 days. The profile that allowed longer has been withdrawn by the authorities themselves. If a reseller is still offering a three-year email certificate, they are selling something that cannot be issued. We removed ours.
Lose the private key and the encrypted mail goes with it
Encrypted messages can only be opened by the key they were encrypted to. Replacing a lost or expired certificate does not recover them, because the new key is a different key. Export the PKCS#12 backup on the day you get the certificate, and keep it after expiry — an expired key still decrypts old mail even though it can no longer sign new.
The certificate belongs to one address
It carries the mailbox it was issued to. A second address, a shared team mailbox or an alias needs its own certificate. If you change employer, or your surname changes, the certificate cannot be edited — you order a new one.
Signing is not the same as Adobe trusting the signature
Worth repeating because it is the most expensive misunderstanding on this subject. A personal certificate produces a valid PDF signature that Adobe Reader will still flag, because the issuer is not on the Adobe Approved Trust List. Only a document signing certificate clears that.
Questions we get asked before people order
Which one should I actually buy?
For most people, a mailbox-validated certificate, and CPAC Basic is the one we sell most of — issued in minutes, and on a range that runs up to organisation validation if you need that later. Go straight to the individual level if a counterparty has to see your verified name inside the certificate, or the organisation level if they have to see your company’s. Go to Certum if price is genuinely the deciding factor: it is the same public trust for about a third of the money.
Do the recipients need anything installed?
No, and this is the single most useful thing about S/MIME. Every mainstream mail client already trusts these certificate authorities and verifies your signature automatically. For someone to send you encrypted mail they need your public key, which they get simply by receiving one signed message from you — most clients pick it up silently and offer encryption from then on.
We already have SPF, DKIM and DMARC. Is that not the same thing?
No, and the difference is the reason invoice fraud still works against organisations with clean DMARC reports. Those records authenticate the sending domain to the receiving server; they are invisible to the person reading the message, they frequently break on forwarding, and they are no obstacle at all to an attacker who registers a lookalike domain and configures it properly. S/MIME authenticates the human and the content, and shows the result to the reader. They solve different problems and you want both.
What is the difference between the Comodo and the Sectigo certificates here?
Technically nothing worth paying for either way: Comodo CA was renamed Sectigo in 2018, so both come off the same infrastructure, from the same certificate authority, and verify identically everywhere. We list both because the names are searched and specified differently — buy the Sectigo one when a policy document, a procurement list or an auditor names Sectigo, and the Comodo one otherwise.
Can I use it on my phone as well as my laptop?
Yes, on as many of your own devices as you like, at no extra cost. Install the same PKCS#12 backup file on each. iOS takes it as a configuration profile and Android through the security settings; both then sign and encrypt from the built-in mail app.
Can I mix brands and levels across a team?
Yes. Nothing about S/MIME requires everyone in an organisation to hold a certificate from the same authority, and signatures verify across them with no configuration. A common and sensible pattern is a cheap mailbox certificate for general staff and an organisation-validated one for the few people who sign contracts.
Is a two-year certificate better value than one year?
Usually, and the term switch shows you by how much per year. The argument for a single year is that a certificate cannot be edited: if your address, your surname or your employer might change inside two years, the shorter term costs less than a wasted one.
What happens when it expires?
Signatures you made while it was valid stay valid — that is what timestamping is for. You order a fresh certificate and install it. Keep the old key file even so: you still need it to read encrypted mail that was sent to the old certificate.
Does it work with Microsoft 365 or Google Workspace?
Yes. S/MIME is a property of the message rather than of the mail platform, so it works through any provider. Outlook on Windows, Mac and mobile supports it directly. Gmail’s web client supports S/MIME on Workspace Enterprise plans, and on other plans you sign from a desktop or mobile client such as Outlook, Apple Mail or Thunderbird instead.
What if I collect it on the wrong machine?
Tell us before you do anything else. Collection generates the key in the browser you collect from, so collecting on the wrong machine is the usual cause of a certificate that will not import anywhere. It is fixable, and it is very much easier to fix on the day than a month later.
Looking for something next to this?
Email signing is one of three things people mean by “a signing certificate”. These are the other two, and the certificate for the server your mail passes through.
- The same range, brand by brandComodo, Sectigo and Certum side by side, if you already know which authority you want.
- Signing documents and softwareAdobe-trusted PDF signatures, code signing certificates and Indian DSC on a token.
- Securing the mail server itselfExchange, autodiscover, smtp, imap and webmail on one multi-domain certificate.
Tell us who has to trust the signature
That one sentence is usually enough for us to name the cheapest certificate that satisfies them — and to tell you when the answer is a mailbox certificate for two people rather than a rollout. No obligation and no sales call.