The S/MIME support was already requested here Problems with mixed S/MIME - PGP Environements - #3 by Chucky
Currently the DC supports Autocrypt so two users with DC can use the end to End encryption. The Autocrypt is also supported by Thunderbird and two Android clients: FairMail and K9. They all also support OpenPGP and S/MIME. But in the FaitMail the SMIME is a paid feature and K9 didn’t implemented yet.
Given that the SMIME is more widely supported (for example by Outlook) it would be very important to support it. This includes many tasks:
- Hide smime.p7m and smime.p7s attachments for email because they are technical files and confuse users. The K9 hides them even while not supporting the SMIME fully.
- Verify signature and show a badge with a certificate.
- Allow to send signed message
- Encrypt message. For this we need to store certificates of contacts if received a signed message previously.
- Manage certs: this is probably can be delegated to OS
- Generate a self signed certificate: it won’t be shown in a badge as trusted but still can be used to encrypt messages.