Hello there. I recently did a clean install of ubuntu following one of those perfect server guides with ispconfig and everything went smoothly. However, postfix is exhibiting some peculiar behaviour.

I have postfix configured to send mail out on port 587 via smtp because I'm on comcast and that's what they require. I can receive mail fine. I can log into my accounts via thunderbird and receive my mail through that program. However, I cannot send mail via thunderbird but I can send mail via webmail (squirrelmail). When I attempt to send mail via thunderbird I get this error:
Sending of message failed.
The message could not be sent because the connection to SMTP server <snip> was lost in the middle of the transaction. Try again or contact your network administrator.
In /var/log/mail.log I see a report that no authentication attempt was made.

How can I resolve this issue?