Matej Sustr writes:
and section 4.2.1/4yz which says:
...replies are 4yz if they can be successful if repeated without any
change in command form or in properties of the sender or receiver
(that is, the command is repeated identically and the receiver
does not put up a new implementation).
It says "can be", not "will be in the foreseeable future".
For example, suppose the spool is full. Probably that problem will pass
quickly, but it doesn't have to. It depends on what filled the spool. The
"4xy my spool is full" response therefore doesn't guarantee that the spool
will have free space in the foreseeable future.
A robust SMTP client should try another MX on 4xy.
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