ned+ietf-smtp(_at_)mrochek(_dot_)com writes:
But this doesn't mean that you should do this: Nothing prevents
someone from having an MX for foo.example.com pointing to a
completely separate mail.example.org while having a SMTP server
running on foo.example.com that silently eats everything sent to it.
And yes, such a setup would be stupid and dangerous, but people do
stupid and dangerous things all the time.
I've seen this exact setup, in two varieties.
workstation15.example.com IN A 10.0.1.15
IN MX 0 smarthost.example.com.
Who knows what you might reach at 10.0.1.15?
The other variety has a proper IP addres for workstation15, and
workstation15 runs a vendor-standard SMTP server, delivering to
somewhere that noone ever reads.
I agree with Ned: Don't fall back to A unless you know there's no MX.
Arnt