ASP: Have tried to set up your Sendmail / Postfix to talk with youre ISPs SMTP?
Maybe its a dirty solution, but you could try to get your Sendmail / Postfix to talk to your ISP, and then make glFusion to talk to the local SMTP..
glFusion -> Local SMTP -> ISPs SMTP. It isnt a longterm solution, but maybe it solves the issue right now for you.
Let me know if you need help to set up the local SMTP to relay mail.