Cannot get mail to work

I just installed mail server and horde.
Installation went well and I created mail domain and addresses for two users.
Horde comes up correctly and I can login, but when I send mail (for now just internally), it shows in the sent folder on horde but never reaches the recipient. I also tried to send mail with the mail command from terminal and it appears to go but does not arrive.
Where can I start looking for problems?

Please first have a look into /var/log/mail.log.
There you will find entries with errors, why delivery did not work.

Did you configure Postfix and Horde in the Univention registry?

Please also take care, that all users have valid e-mail-addresses in the account setings and may access mail.