Hi, the submit.cred file is NOT a new (broken) Apple option. It has been introduced, instead, with the new versions of PostFix, so you could find it also on Linux or BSD installatios running the latest version of it.
It has been introduced to match the MTA with the RFC4468 (http://tools.ietf.org/html/rfc4468) which describes an extension of the IMAP protocol.
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" >> /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" >> /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. You should also fine-tune your postfix configuration in order to prevent it to deliver spam or provide security breaches.