Although full S/MIME support would be great, in my opinion, the most important part is to be able to validate S/MIME signatures. Please vote for https://protonmail.uservoice.com/forums/284483-proton-mail-calendar/suggestions/50377695-s-mime-signature-validation-support-for-e-mail-cli to get this crucial feature into Proton Mail.

Mandatory for me. Most of e-mails are encrypted with S/Mime certificates. Proton users could be able to choose either PGP or smime.P7m.

Received my S/MIME certificate to import it in my Ultimate account just to be dissapointed Proton does not support S/MIME words cannot discribe how frustrating it is to be a customer from a brand wich promotes secure Email communication around the world 24/7 and is not able to deliver the number one feature in secure email communication

S/MIME support Please!

becomes more important now also with gmail rolling this out https://www.theregister.com/2025/04/01/google_e2ee_gmail

Many Government organizations and enterprises use S/MIME and we would be very nice for Proton Mail to have native support for it.

Though we sure would prefer to use PGP, some companies, professionals and governments only accept S/MIME for digital signature and/or encryption, making this feature absolutely necessary.

Many partners use S/MIME. Please implement this feature.

Ich möchte meiner Stadt verschlüsselt schreiben, sie unterstützt aber nur S/Mime.

Might be worthwhile to generalize this suggestion to: support multipart encoding of email. The underlying issue is that Proton destroys other parts of the message, whether text/plain or multipart/signed (S/MIME) in favor of text/html, so if you import email from another service, other encodings are lost/destroyed.

Doing business requires S/MIME in many countries (PGP is not easily verifiable by others as S/MIME is). I have been using Visionary for many years, but recently have had to look around for an alternative precisely because of the lack of S/MIME. I think the additional implementation of S/MIME will only benefit Proton (convince more users to stay and join).

In my country e-government applications and digital identity applications supports only S/MIME. That's why I still have to use Thunderbird and ProtonMail Bridge.

My bank uses as well S/MIME and their IT has limited flexibility...

Implement the signature verification at least. Please.
My bank emails are S/MIME signed and I cannot verify them, even if I download the signature attachment. It seems to be Protonmail is screwing up the email content.
I believe that being able to verify a bank email is a critical security feature that should be implemented ASAP.
Thank you.

And decrypt and certify the signatures as well.

@The Proton Team

Would it be possible to consider this one ? It has been submit 7years ago.

Thx

Please add this feature:)

Definitely this would be great. it's 2023.

PGP pretty useless, not supported out of the box by mail email clients. Whereas S/MIME is by a large number, just to mention: Microsoft Outlook, Mozilla Thunderbird, The Bat!, ios Mail, Macos Mail, IBM Lotus Notes, Blackberry

I appreciate all the work Proton has done with OpenPGP, but regardless we still need S/MIME capability as it is widely used and to verify if incoming emails are authentic or not.