Issues with sendmail, tacking on another domain name, etc.
You want to manually test you mail host, from the command line.
Whilst debugging sendmail issues, came across this great test.
You can run this from the command line. See example.
telnet mailrelay.domain 25
.... answer from mail relay .....
mail test from unix
[tags]sendmail debugging, debugging sendmail, sendmail, debugging, UNIX Coding School[/tags]