That A record MUST point to the SMTP Client.
And the IP address of the SMTP client MUST resolve back to the hostname.
That may not be possible with multi-homing.
out-mail.example1.com A 192.168.0.10
out-mail.example2.com A 192.168.0.10
10.0.168.192.in-addr.arpa PTR out-mail.example1.com
10.5.0.10.in-addr.arpa PTR out-mail.example1.com
22.214.171.124.in-addr.arpa PTR out-mail.example2.com
It's *possible* to set it up. The biggest problems you hit are:
1) A *lot* of software doesn't understand that PTR records can be
2) An even more incredibly large number of sites don't do DNS-over-TCP or
so if the list of PTRs ends up over 512 bytes, things go pear-shaped quickly.
My idea was to distinghuis two differend kinds of SMTP-clients.
Clients that are conforming and should be allowed to send emails to any
Mailserver that has MX records pointing at him.
And Clients that should use their Submission servers. (not fully capable
There are rules for internet hosts, that they should have A and PTR records.
(But to be honnest I could not found the RFCs in which this is spelled out)
And this is just a reminder of that, fully conforming SMTP-clients ARE internet
host and therefore should conform to the rules of internet hosts. (As fas as
they are resonable, so that is open to discussion on this forum)
I also want that RFC2821bis in its main sections only describes the interaction
between fully capable systems.
But i agree in the (new) section about limited capable systems this
requirements should be mitigated.
Hope this makes it clear.
And I hope somebody can give me references to the RFCs where these requirements
are spelled out.