From outside your network e.g at work, open up command prompt and type
telnet domain.com 25
You should receive a response
220 domain.com ESMTP qpsmtpd 0.31.1 ready; send us your mail, but not your spam.
Type quit to exit
This will prove that your forwarding of port 25 on the router is working correctly.
What do the qpsmtpd logs say? Are the emails being blocked for some reason.
Do you have a MX record set up for your domain. You theoretically shouldn't need one but it definately pays to these days.
Have you checked the admin account to make sure the emails aren't being delivered there.
If the email is undeliverable you should eventually get a bounce message back after 4-7 days to say the message was undeliverable.
Jon