now I solved that problem that some user had too ...
the problem was the fqdn!
as I checked:
hostename
result: mailer2
as I checked:
hostname --fqdn
result: empty
so I had to edit the /etc/hosts correctly:
# IP-Address Full-Qualified-Hostname Short-Hostname
right!!! 192.168.10.123 mailer2.domain mailer2
wrong: 192.168.10.123 mailer2
192.168.10.123 mailer2.domain