The short answer.....yes it is possible but I have a question for you.
On your server since only port 80 is open, I assume it is a web server. Correct?
Are you running FormMail on your server? If so, there are serveral exploits where FormMail.pl can be used to spam from the server. This will cause all returns to point to your web server. Post here and let me know if I am on the right track.