UBB.threads PHP Forum Software Community Forums UBB.threads Version 7/8 Support Submit not working / hanging

It seems to be the mail server?
I'm running IIS on Windows 2003 64-Bit

I can send emails by telnet, but UBB threads cannot send emails. I have configured the smtp options in php.ini correcly (server and port).

Interestingly, blat also fails to send emails from this server, wit the "message not accepted" error message. But I can send emails by telnet. Any idea?