Another alternative is to split the envelope: if you know at RCPT TO time
that the addresses have different filtering policies which might lead to
unwanted bounces later on, 450 any subsequent addresses that have
stricter policies than the first one. The downside is that this can lead
to undesirable delays especially if the envelope is large.
How reliably does that work? If I were writing an MTA I'd think a mix of
2xx and 5xx is likely but 2xx and 4xx is unexpected.
Regards,
John Levine, johnl(_at_)taugh(_dot_)com, Taughannock Networks, Trumansburg NY
Please consider the environment before reading this e-mail. https://jl.ly
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