Do I really need an MX record? (for e-mail to work)

[Barry Margolin]

In article <doc7eg$1jbh$1 at sf1.isc.org>, sm5w2 at hotmail.com wrote:

> Because of a recent change to our internet connection, I discovered
> that our MX record has been essentially non-existant for the past 2 or
> 3 weeks.  Our A record is fine.
> 
> It's easy enough to configure our MX record, but I'm reluctant to do so
> because:
> 
> 1) We are getting e-mail, and there is no indication that anyone trying
> to send us e-mail is encountering any difficulty
> 
> 2) The lack of an MX record seems to result in a fall-back to the A
> record (which explains why we are getting e-mail).

That's what's supposed to happen.  MX is an override, it means "instead 
of sending directly to the host in the email address, send to this host."

 
> 3) I've noticed a huge reduction in the amount of "zombie" or trojan
> spam being received by several of our well known addresses.

I wouldn't be surprised if some of the spam cannons don't properly 
implement the protocol.

> 
> Because of all three of the above (especially item 3) I think I'll be
> letting this experiment go a while longer and see if not having an MX
> record turns out to be a great way to prevent spam from finding us.
> 
> Anyone care to comment?

It's generally recommended to have MX records.  I've heard anecdotal 
tales of SMTP implementations that don't properly fall back to A 
records, although I don't really have much sympathy for them.  There's 
also a small DNS performance benefit, as it means only one lookup (MX) 
rather than two (MX followed by A).

> ps:  There seems to be no SMTP-specific newsgroup for talking about the
> benefits and pitfalls of not having an MX record (and no, NANAE does
> not seem relavent).  Are there any web forums for SMTP discussions?

There's a comp.mail.* hierarchy.

-- 
Barry Margolin, barmar at alum.mit.edu
Arlington, MA
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