Look in /var/log/qmail.
Do a ls -la, and tail -f the last file. Then try sending a message to an account on that box and see what qmail reports. Chances are it's a permissions issue with your user directories, have you changed anything recently?
Logfiles are definately the first place to look. Once you isolate an error message I'd try Googling that error and see if there are any fixes. Keep in mind that SME/E-Smith is based of RH, and uses standard daemons like qmail, so many things applicable to other distributions are also applicable to E-Smith/SME.
Hope this helped,
Nathan