OK. So your listener is working. So /rc/bin/service/tcp25 is the next place to look.
What does it say?
You could put 'echo tcp25 running >>/sys/log/whatever' before the smtpd line
to prove that tcp25 is executed. It's likely though that there is a problem running upas/smtpd.
If you read the code (src smtpd works well) you will see that before smtpd writes to the 
/sys/log/smtpd file it checks the incoming IP address to see if it is banned. It then opens the
log file. If that fails then you get no logging. Go play! Take advantage of having the source code.

Nigel