Question : 451  Requested action aborted: local error in processing SMTP Error

What are the reasons why I would get the following smtp error code while trying to send email.

451  Requested action aborted: local error in processing  

Answer : 451  Requested action aborted: local error in processing SMTP Error

Sounds like a duff SMTP connection to me. If it isn't happening with just a couple of email messages then it will be difficult to track down.
If you have anything that is scanning SMTP traffic, firewall, SMTP scanner etc, then they can cause this sort of problem. The Cisco PIX is notorious for it.

Simon.