[Samba] How do you join a Linux mailserver to a Samba domain?

[Steve Ankeny]

I'm running Samba 4.1.6, Ubuntu 14.04 with Dovecot, Postfix & SOGo

When I try to connect to the mailserver via Thunderbird, it gives me a 
specific error --

Unable to open the summary file for Inbox. Perhaps there was an error on 
disc, or the full path is too long.

I've looked at (and reset) file permissions on the local drive where 
mail is stored but not the mailserver, and I'm wondering whether the 
mailserver needs to be identified in Samba somewhere (AD MMC)