[horde] Authentication via IMAP backend and restriction of users

[Florian Schmidt]

Hi all,

suppose the following setup for horde:
- Authentication is done with IMAP authentication ($conf[auth][driver]) and
the IMAP server of one of the big domain providers is used (e.g. 1und1,
Strato and so on)
- as username the complete email address hast to be used as I am using
different domains.

This setup makes it easy to use Horde with hostied domains and email
services by the providers like 1und1, Strato etc.
But with this setup I face the problem, that potentially anyone with a
1und1, Strato etc. login could use my Horde instance.

So my question is: Is there a way to restrict the usernames of type
user at domain.tld which are allowed to login? I can't find a way yet. Any
hints are welcome.

Best regards
Flo