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Date:      Tue, 5 Jan 2016 11:12:38 -0500
From:      Jon Radel <jon@radel.com>
To:        freebsd-questions@freebsd.org
Subject:   Re: sendmail config help
Message-ID:  <568BEB76.9040403@radel.com>
In-Reply-To: <568BD595.6070500@FreeBSD.org>
References:  <201601051413.u05EDiRX065867@mech-as222.men.bris.ac.uk> <568BD595.6070500@FreeBSD.org>

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On 1/5/16 9:39 AM, Matthew Seaman wrote:
>   In fact, even if your mail host uses just the domain as its hostname,=

> I'd still have
>
> z.com. IN MX 0 z.com.
>
> Which looks entirely redundant, but the reason to have it is more socia=
l
> and reputational rather than technical.
Though if your DNS caching and negative caching times are "normal" I=20
believe that's also liable to save time if anyone ever sends you more=20
than one e-mail.  In other words, a sending SMTP daemon will [or is=20
supposed to-YMMV for various spam generating engines] check for an MX=20
record before it falls back to using the A record directly, and an MX=20
record is liable to be cached near the sender for longer than the=20
information that there is no MX record.

Actually, under many circumstances, time will also be saved on the first =

e-mail, as the initial query for MX records will frequently trigger a=20
reply with the appropriate A records included in the additional records=20
of the response, so the sending daemon won't have to make further=20
queries for the appropriate A record(s).

Eventually those milliseconds add up.  :-)

In other words, unless there is good reason to not have an MX record,=20
I'd suggest always having an MX record for any name at which you wish to =

regularly receive e-mail.

--Jon Radel
jon@radel.com


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