Github user sebbASF commented on the issue:
https://github.com/apache/incubator-ponymail/issues/403
It looks like lists.a.o is running different code as it does not add
Message-Id if it is missing; see for example:
https://lists.apache.org/api/source.lua/228334f32b8e893f97f24039508d10133ef3ed537da2edc688dc8258@936409844@%3Cgeneral.jakarta.apache.org%3E---
