Problem
The UCS server can not send emails.
Environment
There is a firewall between the UCS server and the Internet.
In /var/log/mail.log you might see entries similar to the following:
Oct 28 10:17:47 ucs postfix/smtp[1437]: connect to mx01.gmx.net[212.227.17.5]:25: Connection timed out
Oct 28 10:18:17 ucs postfix/smtp[1437]: connect to mx00.gmx.net[212.227.15.9]:25: Connection timed out
Solution
Make sure your firewall is not blocking port 25 outwards. You can verify the remote port is accessible through the telnet command:
root@ucs:~# telnet mx01.gmx.net 25
Trying 212.227.17.5...
Connected to mx01.emig.gmx.net.
Escape character is '^]'.
Another possibility to verify is the use of traceroute:
root@ucs:~# traceroute -n -T -p 25 mx01.emig.gmx.net
traceroute to mx01.emig.gmx.net (212.227.17.5), 30 hops max, 60 byte packets
1 192.168.20.10 0.314 ms 0.253 ms 0.218 ms
2 80.157.204.86 21.330 ms 21.989 ms 21.978 ms
3 212.227.122.16 25.646 ms 25.557 ms 25.675 ms
4 212.227.17.5 24.264 ms 24.054 ms 23.759 ms