Another setting worth looking at. I
From CWIS:
Properties>Services>Email>Setup>Security
Make sure domain filtering is off, i found a random printer when i first started at my company that i couldnt get email to work on and it was because the person previous to me added our domain as the only allowable domain.
If you have a similar model it might not be a bad idea to create a clone file and install it on the machine that is giving you issues.
Just curious, what software version are you running?