It may have to do with squatted/parked domains and/or port 25 being closed
by a firewall. Parked domains generally have A records for showing ads but
often do not have MX records (or an open port 25). Instead of getting a
bounce immediately, you may have to wait for a timeout when the sender
makes a simple typo.

And then there are cases when parked domains do have port 25 open and/or an
MX record. Different discussion.


David Hofstee
OpenText

2017-04-19 3:50 GMT+02:00 John Levine <[email protected]>:

> In article <[email protected]> you write:
> >However, under RFC 5321, section 5.1, I had always thought that without
> >an MX, the system should revert to using the A record as an implicit MX.
>
> You are correct.
>
> On the other hand, these days domains that expect mail generally do
> have MX records, so I can't blame them for treating the lack of MX
> as a signal that the address is bogus.
>
> R's,
> John
>
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>



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