On Fri, 2021-02-26 at 12:03 -0700, Grant Taylor via NANOG wrote:

On 2/26/21 11:46 AM,

borg@uu3.net

 wrote:

Well, I bet my legacy system will bounce it for example...

What specifically is the bounce?

I thought the purpose of the Null MX was to do two things:

1)  Provide as an MX that can't be connected to.

2)  Serve as a signal to things that know how to interpret it that no

mail is to be expected.

I would expect that some server, if not the MSA, /would/ generate a

bounce /because/ the email to the domain is undeliverables.

Exactly. Postfix bounces it immediately with an accurate message:

Domain ???.com does not accept mail (nullMX)

This seems preferable to waiting hours or days for a bounce due to not being able to connect to the A record on port 25 or w/e.