New: Posteo S/MIME add-on available now

Created at 11.February 2026, 12:30 | Category: Info

Update: The Addon is availabe for Firefox and Chrome

Dear customers,

We have some news: We have released our new, easy-to-use end-to-end encryption with S/MIME in Posteo webmail. From now on, it can be used with the AI-free Firefox alternative, Waterfox. The add-on for Firefox is still waiting to be verified by Mozilla. Support for additional browsers and mobile devices will soon follow (we will then update this post in each case).

You can sign, encrypt and decrypt your emails with S/MIME with the help of the add-on directly in the Posteo webmail interface.
Our Posteo development team has developed its own add-on for this purpose. As of now, the add-on has had two security audits performed by independent security experts (Cure 53). It is now available for download directly in your Posteo account settings and can be used with the Waterfox browser in Windows, macOS and Linux. You can find it in the settings via the menu point “PGP- and S/MIME-encryption”.

We place a lot value on integrated encryption being accessible for everyday use – and it also being connected seamlessly into the familiar webmail interface. We as a provider do not save your private key on our servers in the process: Your certificates (including private key) remain on your device locally at all times. This is the only way to ensure true and reliable end-to-end encryption.
Users should be able to have full control themselves of the keys for their communication needs – and not have to trust their provider. This contributes to digital self-determination. With our add-on, we want to show that end-to-end encryption can also work easily and seamlessly in the webmail interface without saving keys on a provider’s server.

We have offered inexpensive S/MIME certificates already since 2024 for your Posteo address(es). They can be acquired with ease in your account settings with just one click (365 cents per year) and they can be used immediately. For a long time, creating certificates for private users was comparatively complicated and cost-intensive even though S/MIME contributes significantly to secure email communication, regardless of provider.
S/MIME is already natively supported in virtually all email clients (also on mobile devices). By being able to also use S/MIME securely with Posteo webmail we want to reach everyone who loves our webmail interface as much as we do.

The plug-in imports the certificates of your communication partners automatically as soon as you click on an S/MIME-signed email. Additionally, it searches for published certificates in the Posteo key directory and uses them automatically when writing to someone whose certificate you do not yet have. Keys are also searched using S/MIMEA.

Here’s what S/MIME can do

S/MIME is a globally used standard for end-to-end encryption and for signing emails that is already integrated in many email clients and apps like Outlook, Thunderbird or Apple Mail. S/MIME certificates issued by a certificate authority are displayed as trustworthy in these programs. This is how you can tell if an email actually originates from the respective sending address. Only you need a certificate for signing your emails: For encryption, your communication partners must also use S/MIME.

By the way, our add-on can not only be used with S/MIME certificates aquired at Posteo, you can also use certificates obtained elsewhere with us.

At Posteo, S/MIME signatures are also checked and the result of the check is displayed in the webmail interface and web app without using the add-on. In the meantime, quite a few large companies and some authorities are signing their emails with S/MIME so that you can recognize the authenticity of an email at a glance.

You can learn how to install and use the S/MIME add-on in our help section:
https://posteo.de/en/help/end-to-end-encryption-with-smime-in-posteo-webmail-interface

Best regards,
The Posteo Team