As Klaus said, it is perfectly valid for any domain to not have MX
records - instead it relies on the A fallback for email deliveries.
Many versions of Exchange will also try the A fallback, regardless of
the existence of MX RR, when all specified MX servers defer. This should
be considered a bug, but it must also be compensated for.
In the RFCs the A fallback is called "an implicit MX RR, with preference
of 0, pointing to that host." (RFC 5321)
When the MX RR would just be duplication of the A RR, some admins don't
see the need to add MX explicitly.
On 21/01/15 22:13, John Schmerold wrote:
> We have had various troubles with sender verify, however I am not sure that
> any of my users would care to receive messages from a domain without some
> basic means of receiving a reply. At this point, I am not considering
> whether the mx record is valid, but I would like to block domains without
> one. The RBLs are strugling to keep up with all the random "innovative"
> domains, this seems to be a good tool in the anti-spam arsenal.
>
>
> On Wed, Jan 21, 2015 at 5:30 AM, Jeremy Harris <jgh@???> wrote:
>
>> On 21/01/15 11:18, Klaus Ethgen wrote:
>>> There are many legit senders without MX
>>> record. That is fully ok and there are mail admins out there that do not
>>> see any reason why to add A and MX with the same content.
>>
>> How do they receive bounces?
>> --
>> Jeremy
>>
>>
>> --
>> ## List details at https://lists.exim.org/mailman/listinfo/exim-users
>> ## Exim details at http://www.exim.org/
>> ## Please use the Wiki with this list - http://wiki.exim.org/
>>
