The following list describes some situations that you can avoid or easily fix.
If mail is not being delivered to the expected address, check the user's .forward file. The user might have put the .forward file in the home directory of host1, which forwards mail to user@host2. When the mail arrives at host2, sendmail checks for user in the NIS or NIS+ aliases and sends the message back to user@host1. This routing results in a loop and more bounced mail.
To avoid security problems, never put .forward files in the root and bin accounts. If necessary, forward the mail by using the aliases file instead.