Github user sebbASF commented on the issue:
https://github.com/apache/incubator-ponymail/issues/182
The Subject: header is optional according to RFC822.
For example the issues@ponymail list allows the Subject to be omitted
(unlike dev@).
Sample test message:
http://my.ponymail/thread.html/5cbfdece07502461f9645d77038fe867c08073a324e25a02d556d5b9@%3Cissues.ponymail.apache.org%3E
This was sent on Oct 27 2016:
https://lists.apache.org/[email protected]:dfr=2016-10-27|dto=2016-10-27:
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