[horde] Can't send invitation confirmation mails using an horde activesync account

[Wolf Moestl]

Dear list readers,


I'm struggeling with some strange behaviour:

I'm using the activesync addon for synchronizing mails, contacts, notes and
calendar entries on my android phone (Android 5).
I created an "Microsoft Exchange Active Sync" account. Sending and
receiving mails for this account works as expected, so far everything is ok.

But if that account receives an appointment invitation it can't send  the
confirmation mail. I just get the "Can't send" error on my android, which
is strage as the user can send regular mails.

I checked the log files in /var/log - nothing special here.

My setup:

   - Debian Wheezy
   - Plesk 12.0.18 Update #47
   - Postfix Mail Server
   - Courier IMAP/POP3 Server
   - Horde 5.2.1


Any suggestions what I can check not narrow down the issue?

Thanks a lot!

Cheers,
Wolf