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 <[email protected]> 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/ >> -- ## 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/
