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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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[-- Attachment #1 --]
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 social
> and reputational rather than technical.
Though if your DNS caching and negative caching times are "normal" I 
believe that's also liable to save time if anyone ever sends you more 
than one e-mail.  In other words, a sending SMTP daemon will [or is 
supposed to-YMMV for various spam generating engines] check for an MX 
record before it falls back to using the A record directly, and an MX 
record is liable to be cached near the sender for longer than the 
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 
reply with the appropriate A records included in the additional records 
of the response, so the sending daemon won't have to make further 
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, 
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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