We already reject email replies to public topics via
SiteSetting.disallow_reply_by_email_after_days and raising the
OldDestinationError. This PR introduces similar behaviour for group inboxes, but without the rejection, and only when SMTP is enabled for the group.
If a reply is sent via email and the post is older than
SiteSetting.disallow_reply_by_email_after_days days ago, then we create a new topic instead of making a reply in the old one and link back to the original topic. This is done to prevent long running group inbox discussions. The new topic looks like this: