Postfix Not Accepting Commands on OSX Lion - Timeout Occurs
The last three log messages turned out to be the culprit. Googling for them turned up this page;

In order to fix the problem your console is displaying, you should:


Create a submit.cred file in /etc/postfix directory (sudo touch /etc/postfix/submit.cred will do).
Create a header for that file: sudo echo "submitcred version 1" &gt;&gt; /etc/postfix/submit.cred
Add as many lines as the number of users you want to enable to submit mail, in the form hostname|user|passwd: sudo echo''hostname|username|password" &gt;&gt; /etc/postfix/submit.cred
Give the file the appropriate permissions: sudo chmod 600 /etc/postfix/submit.cred


Once you've done this you could restart postfix and everything sould run smoothly.

I created the file /etc/postfix/submit.cred and set the permissions to 0600. Everything started working after that.