The pre-installed email certificates can be viewed at "Settings > Certificates > Email certificates". It is possible to add more certificates using the command line (as the UI support is missing).
1. Copy the certificate from PC to device.
scp JSCCinternalCA.crt email@example.com:
2. Copy certificate to "/etc/pki/ca-trust/source/anchors" folder.
[root@Sailfish nemo]# cp JSCCinternalCA.crt /etc/pki/ca-trust/source/anchors
3. Run command update-ca-trust
[root@Sailfish nemo]# update-ca-trust
4. Open Email certificate from settings and check that added certificate "JCSS internal CA" can be found in the list.
Settings > Certificates > Email certificates