Editing MX record (section)

==Handling of delivery failure==
The SMTP RFC<ref name="RFC5321"/> is ambiguous about exactly what kinds of delivery failure must result in re-attempting delivery via more distant MX records (those with higher preference values).

When servers indicate temporary failures, either by explicitly sending a 4xx error or by ending the connection unexpectedly (which must be treated as a 451 error, according to [http://tools.ietf.org/html/rfc5321#section-3.8 Section 3.8] of the RFC), [http://tools.ietf.org/html/rfc5321#section-4.5.4.1 Section 4.5.4.1] says:
{{Quote|The sender MUST delay retrying a particular destination after one attempt has failed.}}

However, when the sender retries, the RFC is silent about whether this should be to the same server, or a more "distant" MX record. It does say, in [http://tools.ietf.org/html/rfc5321#section-5.1 Section 5.1]:
{{Quote|When the lookup succeeds, the mapping can result in a list of alternative delivery addresses rather than a single address, because of multiple MX records, multihoming, or both.   To provide reliable mail transmission, the SMTP client MUST be able to try (and retry) each of the relevant addresses in this list in order, until a delivery attempt succeeds.}}

Some servers (such as [[Sendmail]] and [[Postfix (software)|Postfix]] 2.1 or later),<ref name="postfix20-21">[http://archives.neohapsis.com/archives/postfix/2005-11/0636.html ''If the primary MX responds, but fails mid-transaction, Postfix 1.2 and 2.0 will not try a backup MX.''] {{webarchive|url=https://web.archive.org/web/20090623201411/http://archives.neohapsis.com/archives/postfix/2005-11/0636.html |date=2009-06-23 }}, Re: does not change to mx with lower priority, From: Victor Duchovni (Victor.DuchovniMorganStanley.com) Date: Fri Nov 11 2005</ref> will attempt the next-furthest MX server after some types of temporary delivery failures, such as greeting failures.<ref>A greeting failure is an error-code that is sent instead of or in response to the standard SMTP greeting handshake.</ref> Other servers (such as [[qmail]] and [[Postfix (software)|Postfix]] 2.0 or earlier) will only use more distant MX records if the servers specified in the shortest-distance MX records could not be contacted at all. Despite the difference, both behaviors are valid - since the RFC is not specific.