Actually, maybe not - I over-simplified. (Thanks to John Leslie for
making me be more precise.) RFC 2821 says:
4yz Transient Negative Completion reply
The command was not accepted, and the requested action did not
occur. However, the error condition is temporary and the action
may be requested again. The sender should return to
the beginning
of the command sequence (if any). It is difficult to assign a
meaning to "transient" when two different sites (receiver- and
sender-SMTP agents) must agree on the interpretation.
Each reply
in this category might have a different time value, but the SMTP
client is encouraged to try again. A rule of thumb to determine
whether a reply fits into the 4yz or the 5yz category
(see below)
is that 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.)
So this last sentence would imply a 5xx response for
misconfigured CSA SRV
records, and this tallies with common responses to bad MX
records (if the
MTA software is paranoid enough to check them for RFC1918
addresses etc.).
The 4xx I referred to above can come from lower-level DNS
problems that
prevent a DNS lookup from succeeding (bad delegation, etc.). The exact
meaning of "without any change ... in properties of the sender or
receiver" isn't totally clear because this kind of broken-DNS
4xx response
requires someone to fix something in order to be resolved. But if you
understand "properties" to be restricted to things that are directly
email-related (e.g. MTA software and configuration, MX
records, CSA SRV
records) as opposed to incidentally related (e.g. DNS
delegation) then I
think it makes sense.
Yes? No?
If it were only that simple. One large well known web based mail site will give
a 5xx response if the users mailbox is full. Another large well known web based
mail site will give a 4xx response in the same case.
So, should a full mailbox be considered transient or permanent?
Mike