No port 25 traffic allowed out from Ubuntu Server

I have three Ubuntu 8.04 LTS servers that will not allow outgoing connections on port 25.

For instance, I can do the following from the command line on my server:

telnet anotherserver.com
or:
telnet anotherserver.com 23
or:
telnet anotherserver.com 80

but I cannot do the following:
telnet anotherserver.com 25

I get:
Trying 111.222.333.444
telnet: Unable to connect to remote host: No route to host

This is true on all three 8.04 servers I have, but there is no problem on the 9.04 or 10.04 servers. It does not make a difference if I use sudo telnet. Two of these servers are running Postfix, the third is not (if that matters). I do not see anything in IPTABLES to prevent port 25 traffic, but it does look like port 25 traffic is being selectively blocked or rerouted somehow.

Where should I look next?

Thanks.