back to the beginning …
I found out, that I have to restart the univention-directory-listener service after patching the script, mentioned in the first post.
I prevented modifications to this file via
chattr +i /usr/lib/univention-directory-listener/system/fetchmailrc.py
Hope Univention won’t update this script too often 
Thanks in advanced,
Christian.
If someone needs this patch, too:
# diff /usr/lib/univention-directory-listener/system/fetchmailrc.py /usr/lib/univention-directory-listener/system/fetchmailrc.py.backup
116c116
< flist.append("poll %s with proto %s auth password user '%s' there with password '%s' \"mda /usr/sbin/kopano-dagent %s\" %s %s #UID='%s'\n" % (
---
> flist.append("poll %s with proto %s auth password user '%s' there with password '%s' is '%s' here %s %s #UID='%s'\n" % (
121c121
< new['uid'][0],
---
> new['mailPrimaryAddress'][0],