John Schmerold <schmerold2@???> 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.
Please don't.
It is perfectly valid to have a setup like this:
example.com IN A 192.0.0.2
And use someuser@??? as sender, as long as an MTA is listening
on port 25 at 192.0.0.2
The A record has always been treated as an implicit MX record in the
absence of real MX records and works as such "since the dawn of the
internet".
Rejecting mails because there is no MX record for a domain as long as an
A (or AAAA) record exists is clearly not "a good tool in the anti-spam
arsenal".
On the other hand, you may freely reject mails where the sender domain
neither has a MX or A or AAAA record, since those senders really cannot
be reached via SMTP.
