Idézem/Quoting Matus UHLAR - fantomas <>:

On 10.06.17 14:53, SZÉPE Viktor wrote:
RFC 5321 states in

The lookup first attempts to locate an MX record associated with the name.
... If an empty list of MXs is returned,
the address is treated as if it was associated with an implicit MX
RR, with a preference of 0, pointing to that host.

Were you a ware of that?
I think it is very unusual and dangerous.

Do modern MTA-s - including Courier - implement that?

This behaviour was described in rfc 821 and 2821.
AFAIK all MTAs implement this behaviour since MX records were implemented.

What and why exactly sounds unusual and dangerous to you?

I think it gives us no means to stop emails for a domain.
I thought removing the MX record and not listening on port 25 is enough.

This way anyone my send an email to a mailserver-less sub/domain.

SZÉPE Viktor
+36-20-4242498  skype: szepe.viktor
Budapest, III. kerület

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