I'm having an issue with Proton.me/Protonmail.com. The certificate needs to be self accepted, but the Mail app won't give me the option to accept it. Geary email allows this. So something in the Elementary implementation has taken this away.
- Sign up for a proton.me account
- Download the Proton/Protonmail bridge and sign in
- Follow the directions for Thunderbird from Proton for setting up Elementary Mail.
- Notice that at the step where the user has to accept the certificate there is nothing available from the Mail app. It just fails.
If you download Geary and do the same steps you will be allowed to accept the certificate.