I would like to confirm that it’s possible to make a successful connection from Apple El Capitan IMAP, ICAL and addressbook to SOGo 2.3.10-1 on Nethserver 6.8 release using self signed certificate.
First you have to start with an IMAP account definition for e-mail.
When successful then you will have to accept a self signed certificate. Do it and add it to your Keychain.
Then add a SOGo calendar and addressbook accounts.