> And if it bounces ...
> Emergency Stop Emailing Immediately!
Others have hinted at this, but let me call it out explicitly:
Pay attention to the SMTP Enhanced Status Codes
A [45]XX can be graduated by the associated [45].X.X.
E.g.:
552 5.2.3
552 5.3.4 You sent something too porky for the recipient. That's
*your* fault, not theirs.
452 4.3.1 The entire destination system is out of disk space.
Likely not the recipient's fault.
5XX 5.3.5 The sysadmins running the recipient's mail system are
not top notch. Not the recipient's fault.
550 5.6.9 The recipient is hosted on a server running old software
that fails on UTF8 headers. While they might lose
some of the messages you send their way, this isn't
necessarily an excuse to boot them from the list
outright (unless the majority of your traffic
provokes this failure).
In other words, it would be nice if mailing list managers were more
nuanced in their handling of non-delivery diagnostic responses.
That's not so say you shouldn't punt somebody with an overall delivery
failure rate >80%. But within the failure diagnostics you get back,
pay attention to what the remote system is telling you, and judge
accordingly. Not all permanent failures are permanent.
See
https://www.iana.org/assignments/smtp-enhanced-status-codes/smtp-enhanced-status-codes.xhtml
--lyndon
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