Without this all emails with send-as-user enabled that go to most
large email providers will be classified as spam. Using List-Id is what mailing
lists use.

I've adapted all mail adapters, but am unable to test the amazon one (although
it apparently is just phpmailer compatible) and the mailgun one (but it looks
pretty straight forward, and I cross-checked with their API docs.

Have been running this both with sendgrid and phpmailer on
our llvm setup. The other implementations seem very similar.