Yes, I fixed problems with SMTP on Debian Sarge 3.1
half of the problems were caused by my ISP not by ISPConfig nor Debain.
Just follow my posts, there is one whole thread about it. Same smtp problems were on Fedora and Debian.
As Falko use to say:
"Follow tutorial to the letter...!"
P.S. be sure that you apply all of the mentioned in tutorials. There is one debian package script in which is not correct, so better choice is actually to copy and paste script / config file from Falko's tutorial. Or compare config file / script on your server with example in Falko's tutorial VERY carefully. Especially for saslauthd.
You can even delete and then create new /etc/init.d/saslauthd by copying it from tutorial. Works for me.