Cisco Cisco Email Security Appliance X1070 Information Guide

Contributed by Nasir Shakour and Enrico Werner, Cisco TAC
Engineers.
Jul 17, 2014

What SMTP response codes does the ESA support?

The ESA is fully−compliant with the original  Simple Mail Transfer Protocol (SMTP) specification of RFC
821. The complete list of message response codes are listed in RFC 821.

The 400 series of codes indicate non−fatal errors or responses in the SMTP conversation. Such errors are
often referred to as soft−bounces, as unless otherwise instructed, the transmitting mail server will re−attempt
delivery to the intended receiver at a later time. Soft−bounces can be caused by a variety of things ranging
from a mail account exceeding its quota to SMTP hosts policy limits imposed upon a particular sender.

The 500 series of codes are also error codes. Unlike the soft−bounces of the 400 series however, the 500
denote a complete failure on the part of the receiving server to accept a message for delivery. These failures,
referred to as hard−bounces, are typically caused by such issues as malformed or unknown mail−to addresses,
a failed reverse DNS check, or other receiver host policy limits.

It is important to note that depending upon the policy set for the transmitting mail server, a soft−bounce may
eventually become a hard−bounce

Here is a list of the most common SMTP response codes:

211 System status, or system help reply

214 Help messageInformation on how to use the receiver or the meaning of aparticular non−standard
command; this reply is useful onlyto the human user

220 Service ready

221 Service closing transmission channel

250 Requested mail action okay, completed

251 User not local; will forward to

354 Start mail input; end with <CRLF>.<CRLF>

421 Service not available, closing transmission channel This may be a reply to any command if the service