At 03:19 -0400 on 04/19/2008, Hector Santos wrote about Re: I-D
Note, I am not questioning that an explicit MX requirement method is
not a valid consideration. I am questioning to what purpose?
Just consider the many transactions with addresses such as:
no-reply @ validdomin.com
that many feedbacks system use today, including bad guys and the
bad/good direct marketing people.
What is the envelope address for these messages? So long as it exists
and will accept delivery failure Email, the fact that the FROM
address is SEND-ONLY (but in a valid domain that will respond by
sending a "NO SUCH USER" error message to any recipient that tries to
reply [or send] to such an address) is a non issue that is self
correcting (you spank the user who tries to send to the address).
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).