Hi all,
I have read about this and found some more or less orphaned information, that make me believe, it should be possible. But somehow I am confused about the different contents and i do not make any progress about this.
So I call for help.
System:
Linux Debian ARMBIAN Stretch
APACHE2, php7.2
Nextcloud 18 fresh install
Installed “external Login” App to authenticate using IMAP (dovecot) server.
Current situation:
User can log in using regular e-mail and password, but E-Mail app keeps unconfigured.
Wanted situation:
After Log in the e-mail app should autoconfig and display the mail box of the current user.
Secondary targets:
autoconfig Calender and Addressbook with existing Horde Database.
Any hints or ideas?
And please: Assume You are talking to some regular user. Don’t talk rocket science, even so, your counterpart has more than just “rm -f -r /” knownledge 
br
Carsten