Postfix not accepting unsecured connections on Port 587 after upgrade Plesk to 10.3

Here is the solution

/etc/postfix/master.conf

Scroll down to “submission inet n – n – -……” line.
Change “smtpd_tls_security_level=encrypt” to “smtpd_tls_security_level=may”

I got this from this post.

http://forum.parallels.com/pda/index.php/t-105983.html