We are having and issue with sending some emails to domains outside of our network. We fixed a reverse dns issue and it did not resolve all of the issues.
We then realized that we could not connect via telnet over port 25 to these external mail sites (telnet mail.yourhost.com 25). We can connect from other servers to these external sites via telnet just not from the mail server.
What might be causing telnet not to work from the mail server?
Server – Windows 2003
Email – Exchange 2003
Update
There was a firewall rule setup for RPC over HTTP that was causing the issue. Once the rule was removed everything was restored to normal.
Best Answer
The most obvious answer is a firewall somewhere - have you checked the software filewall on the mailserver, and any hardware firewalls/routers in the way?
Can you telnet to other ports on the external severs? Can you ping/traceroute them?