On Mar 18, 2021, at 10:55 AM, Laura Atkins
MTAs that send high volumes of email do pay attention to the other digits and
the extended codes. In fact, many of them do full text parsing to try and
determine how they should handle future mail to that address.
https://smtpfieldmanual.com is one example.
Is this "paying attention" really happening *during* delivery, to
guide further progress of the SMTP transaction, or, as seems more
likely, rather an after-the-fact forensic analysis to keep lists
in good hygiene.
Detailed SMTP server replies are certainly useful forensically,
what's under discussion in this thread, is to what extent the SMTP
protocol engine in the sending client really needs to be concerned
with such details.
Occasionally, an MTA with an uncertain reputation may choose to
conditionally soften a 5XX reply to a 4XX reply, if it has another
means to reach the destination (from a different IP address, ...).
Perhaps that's the every-day reality for some sending systems, and
in that case I can see these closely scrutinising the server reply.
In most other cases, there's little reason to look beyond the first
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