When a mail server unsuccessfully attempts to send a message to another server, it typically results in an automated email response called a bounce. Bounces may be caused by a block by the mailbox provider (MBP), and the reason and additional information such as a postmaster url is normally included within the bounce code.
Here are some examples of actual bounce messages indicating policy blocks. Bounce messages are classified as hard bounces, which have a 5xx code, or soft bounces, which have a 4xx code:
smtp;554 Message not allowed - [PH01] Email not accepted for policy reasons. Please visit http://postmaster.yahoo.com/errors/postmaster-27.html 
421, "4.7.0", Our system has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users from spam, mail sent from your IP address has been temporarily blocked.
550 SC-001 (BAY004-MC4F3) Unfortunately, messages from IP XX.XXX.XX.XXX weren't sent. Please contact your Internet service provider since part of their network is on our block list. You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors