Quote:
Originally Posted by JeremyNicoll
Wouldn't a rule for:
A header called In-Reply-To: does not exist
help?
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I had forgotten the In-Reply-To (and References) headers. Indeed, while technically optional, almost all mail clients these days will add an In-Reply-To header as recommended in the RFC (see
https://tools.ietf.org/html/rfc5322 Section 3.6.4).