Newbie Question: Is anyone actually using the Null MX (RFC 7505)?

Fri Feb 26 19:03:37 UTC 2021

On 2/26/21 11:46 AM, borg at 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.

> I cant speak about Sendmail, qmail, Exim.. when they started supporting it.

My Sendmail boxes have been dealing with the Null MX just fine.  The 
aforementioned bounce is /expected/ to tell the sender that the 
destination address is bad.

> So, In my opinion changing already working standards in a way
> that they arent full compat with old systems is imo bad aproach.

IMHO there is little, if any, effective difference between the Null MX 
and an MX pointing to an unresolvable name or an non-routed IP.  They 
cause a hard / fast failure in an early upstream MTA thus induce a bounce.

Depending on the MSA, the delivery problem may even be presented to the 
user as they are submitting the message to the MSA.

-- 
Grant. . . .
unix || die

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