Thanks for posting.
I think the best place to start would be when this started happening and what changes you can think of (script, updates, etc.) were made around that time? Is this an internal SMTP server?
Also, are you able to send and receive emails from the SMTP server through other applications? For example, if you created a Mail account on the Mac and sent an email through the SMTP server do you receive it? It may be useful to test this or a Mobile Me scenario to help narrow down the issue.
While sending emails via SMTP was working at one time, here are some other things to re-verify:
- Emails are UTF-8 format
- The Name field is not blank in File > Send > Mail > SMTP