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 cerficiates 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.

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.