Consider an email going through the mail gateway. Its headers References
and In-Reply-To are used in mail gateway to find if it is an answer to an
existing discussion thread.
Notably in mass mailing it is used to set mailing traces (called statistics
before 13.0) as opened and replied. However currently only the References
header is used.
In this commit we now use References or In-Reply-To like what is done in
mail gateway.
== 13.2 FORWARD PORT SPECIFIC ==
Code in tests has been updated to use new mockups tools and variables available
in test_mail
LINKS
Task ID 2257717
PR #51445
Forward-port-of: #51319
Forward-port-of: #51247
X-original-commit: 9921e38692e7dc4547afc34acc395da340eca410
