https://meta.discourse.org/t/improving-mailman-email-parsing/253041
When mirroring a public mailling list which uses mailman, there were some cases where the incoming email was not associated to the proper user.
As it happens, for various (undertermined) reasons, the email from the sender is often not in the `From` header but can be in any of the following headers: `Reply-To`, `CC`, `X-Original-From`, `X-MailFrom`.
It might be in other headers as well, but those were the ones we found the most reliable.
