Trev writes:
First:
In section 8.1 you say that you are using SMTP reply code #421 to express the error condition "this mailbox is not available within this transaction, because of a whitelisting conflict." Unfortunately, a few years after you wrote that, the authors of RFC 2821 co-opted #421 to use for another purpose. The new purpose is quite a severe one: it makes the client believe that the server is being forced to shut down!
If this has not yet caused interoperability problems for you, it's only because widely deployed clients have not yet chosen to detect #421 and implement its RFC 2821 semantics. But they are free to do so, at any time, and when they do you'll have problems.
Perhaps, but I fail to see even a single reason why a client would even CARE about this, particular, marginal, situation. I fail to see anything to be gained from the client perspective from even BOTHERING to check for this situation.
I've made the following statements many times, in different contexts, and it applies here equally well: this is a result of nothing but what you get when you have pointy-headed academicians entomb themselves inside ivory halls of academia, where they draw up grandiose dissertation about how the life outside should be like, and pat each other on the back for being so smart. Meanwhile, their theories bear no relevance on reality whatsoever.
It's no big deal changing the numeric code. But, at this time I see no particular hurry to do so.
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