Getting Proton.me/Protonmail.com bridge configured with Mail?

Question

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.

To repeat:

1. Sign up for a proton.me account
2. Download the Proton/Protonmail bridge and sign in
3. Follow the directions for Thunderbird from Proton for setting up Elementary Mail.
4. 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.

Answer 1

Through some trial and error I was able to connect to the proton bridge using elementary OS' native mail app.

1. Sign up for a proton.me account
2. Download the Proton/Protonmail bridge and sign in
3. Open Proton Bridge and go to Settings -> Advanced settings
4. Click "Export TLS certificates" and save them somewhere (I did documents)
5. Copy the exported cert.pem into your certificates folder sudo cp cert.pem /usr/local/share/ca-certificates/certificate.crt
6. Resync certificates with sudo update-ca-certificates

The output should be similar to

Updating certificates in /etc/ssl/certs...
rehash: warning: skipping ca-certificates.crt,it does not contain exactly one certificate or CRL
1 added, 0 removed; done.
Running hooks in /etc/ca-certificates/update.d...

Adding debian:certificate.pem
done.
done.

After this you should be able to connect to your proton bridge as expected with username/password/STARTLS.