The ability to read signed emails would already be great . It's really annoying when people send you signed emails and you cannot read the content.

+1 (ProtonMail Visionary subscriber)

My government issues me with certificates I can use for legal comms in country and with all local, regional, national agencies. Protonmail can only be my secondary email system while I cannot communicate with gov ( not to say many businesses )

I agree with the comments below, this is needed for business communication and would make this a real Outlook/Microsoft alternative.

I also would like this feature. As other people have mentioned, S/MIME is widely used in business (at least in Outlook) for message authentication and encryption. It would be very nice to be able to seamlessly send encrypted messages to those users, even if we had to provide our own S/MIME certs or something.

Would like to use all my votes on this subject. Are even ready to pay an increased fee for this feature.

Yes please !

I'd also like to have this feature implemented. S/MIME is often used in business so it would really help.

S/MIME would be great for replying to users who sign their emails with certificates securely.

Agree with this feature, I use Protonmail, but do send emails to addresses outside of Protonmail. I want to still sign and encrypt the outbound emails using my S/MIME certificate. At the same time, received emails signed or encrypted with S/MIME I would want to be able to read in Protonmail.

Agree with this feature, I use Protonmail, but do send emails to addresses outside of Protonmail. I want to still sign and encrypt the outbound emails using S/MIME. At the same time, received emails signed or encrypted with S/MIME I would want to be able to read in Protonmail.

I would like to send s/mime certified Mails to other people, so they may see, that this mail is originally from me and not from a spam-machine or something else.

In addition I don't see, if the certificate from a s/mime-sender is really a good certificate or only a file with the correct ending, or something else, i only can see a file.

Implementation of signing and possibly s/mime and/or pgp-encryption is really important, especially for a sender with features like protonmail.

S/MIME is (somehow) the defacto of email encryption/digital signing and gigantic players like apple mail, outlook do support it as well. Other more recent and much better solution is mailfence.com which provides both OpenPGP and S/MIME compatibility.
Protonmail needs to take that into account as well.

If I recive in protonmail signed message, it should store public key to contacts and send to that email encrypted messages.