When we migrated over from AMO to Marketplace we moved over all the users. Which means when we send the FxA email, we really don't want to email those AMO user about FxA, that would suck. The suggestion is to email all users who have logged in since the db split. That was on April 2nd. So let's give ourselves a bit of space and say something like May 1st onwards. That's all users in the last 6 months. So the fxa_mail.py management command should filter on that date and only include users who have logged in since then.

Please add STR here or mark it with [qa-] if no QA is needed.