At 13:32 -0400 on 04/23/2007, David F. Skoll wrote about Re: New
issue (was Re: rfc2821bis-01 Issue 18: Usability of:
However, it's a SHOULD, so a client could treat it as a 4yz if it
really wants. Actually, the more I think about it, the more it might
make sense to treat it as a 4yz if there are any lower-priority MX
hosts left to try, and a 5yz if not. But putting that in an RFC is
kind of ugly; I wouldn't know how to word it nicely.
Why not just say that. IOW: Something on the order of:
If there is a an undefined response code, the client SHOULD terminate
the submission by treating it as a 5yz code. In the event that there
are any lower-priority MX hosts left to try, the client MAY instead
treat it as a 4yz and try the lower-priority MX hosts until the
message is accepted by one of the MX hosts or all have been used at
which point the final undefined response code SHOULD be treated as a
5yz.