Kolab 4.3: Mail not working

[Sophie N.]

Hi there!
I did a fresh install of Kolab as described on
https://docs.kolab.org/installation-guide/debian-community.html but emails
won't work in any direction. To prevent from messing around with external
clients, I tried roundcube webmail.

For example, if I try to send a message from one internal user to himself,
the message isn't saved as sent and it gets a delivery error. You can't see
this in roundcube, but the log tells you

Oct  4 17:45:58 mail postfix/submission/smtpd[1441]: connect from
localhost[::1]
Oct  4 17:45:59 mail postfix/submission/smtpd[1441]: A1AB4B86B1F:
client=localhost[::1], sasl_method=LOGIN, sasl_username=franz.test at xxx.de
Oct  4 17:45:59 mail postfix/cleanup[1446]: A1AB4B86B1F: message-id=<
5716c3010d45f11e01459b67ad4e4aae at xxx.de>
Oct  4 17:45:59 mail postfix/qmgr[1324]: A1AB4B86B1F: from=<
franz.test at xxx.de>, size=343, nrcpt=1 (queue active)
Oct  4 17:45:59 mail postfix/smtp[1447]: connect to
127.0.0.1[127.0.0.1]:10024: Connection refused
Oct  4 17:45:59 mail postfix/smtp[1447]: A1AB4B86B1F: to=<franz.test at xxx.de>,
relay=none, delay=0.59, delays=0.58/0.01/0/0, dsn=4.4.1, status=deferred
(connect to 127.0.0.1[127.0.0.1]:10024: Connection refused)
Oct  4 17:46:00 mail postfix/submission/smtpd[1441]: disconnect from
localhost[::1]

This box runs with Debian 8.2 (also fresh install), has a proper Hostname
but is located behind a NAT.

Any ideas regarding this issue are welcome!

Greetings
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