Cisco Cisco Email Security Appliance X1050 Guía Para Resolver Problemas

Contributed by Mark Vegh and Stephan Bayer, Cisco TAC Engineers.
Oct 14, 2014

Introduction
Symptoms: 421 or 451 Error Codes in Mail Logs
     Google / Gmail
     Yahoo
     Other MTA
What Are Destination Controls?
     Gmail / Google Apps Destination Controls
     Yahoo Destination Controls

This document describes destination control settings for the Email Security Appliance (ESA).

If a large amount of email was sent to a particular email domain in a short time, the receiving servers may 
temporarily defer the email traffic with messages like:

450, "4.2.1" The user you are trying to contact is receiving mail too quickly.

 Please resend your message at a later time. If the user is able to receive mail at

 that time, your message will be delivered. For more information, review this article.

"421 Message from x.x.x.x temporarily deferred" or "451 Message temporarily deferred". 

"451 4.4.0 Primary target IP address responded with: ""421 4.2.1 Unable to connect."

"421 Too many concurrent SMTP connections; please try again later" 

SMTP can be thought of as a conversation between 2 servers. They are self managing in that the recepient can
rate limit your sending ESA. With Destination Controls and block  you from sending more emails until a
specified time threshold has been reached. Proactively rate limit yourself for communication to specific
domains with Destination Controls on the ESA.