Robert A. Rosenberg wrote:
Well, an explicit MX does not resolve this problem of a response
address not being reachable.
You said that "validdomin.com" is a valid domain so attempts to send to
the no-reply address will get to the SMTP server supporting
validdomin.com which will then send a "No Such User" reply (which in my
book qualifies as reachable since the message to no-reply WAS delivered
to the SMTP Server (but then rejected).
My overall point, in so many words, was that mandating an MX record
requirement is meaningless IF the GOAL of the mandate is to thwart bad guys.
So even if validdomain.com has a MX record, it means nothing if the
other two key parts of the SMTP transaction equation are not valid:
- Service Port
- Valid Recipient
At the very least, the service port is a major consideration.
In short, having a MX record does not make you a valid mail host until
everything else about the process is valid.
--
Sincerely
Hector Santos, CTO
http://www.santronics.com
http://santronics.blogspot.com