Many IPS block the use of the standard mail SMTP port (port 25).
In order to deal with this SMTP can be configured to run on alternate/additional ports.
The process for this is fairly simple:
Copy /etc/xinetd.d/smtp_psa to smtp2_psa
In the new file, REPLACE service smtp WITH service smtp2
Then modify /etc/services and add the smtp2 service (port 26 in this example).
smtp2 26/tcp mail
smtp2 26/udp mail
Then restart xinetd
/etc/init.d/xinetd restart
Check that the new port is now listening
# netstat -plunt |grep :26
That should return something similar to
tcp 0 0 0.0.0.0:26 0.0.0.0:* LISTEN 2345/xinetd
Test connection by telnetting from an outside box to port 26 on the server:
telnet 123.123.123.123 26
