You’re right. Seems we need a hook to set it at login. I only found some old code but it should be possible:
https://wiki.horde.org/CustomizingPreferences
https://wiki.horde.org/CustomizingPreferencesH3?referrer=HowTo#toc8
That’s really nice, a must-have.
I also noticed that the mail addresses in turba AD address book are missing, only usernames are synced.
I am going to add/fix these points asap, thanks for testing.