Gmail to MXroute when Google threatened to pull the grandfathered free Gmail custom domain thing. Got their lifetime plan, easy enough to configure so outgoing mails don’t get marked as spam. However, the major downside is it’s still using Spam Assassin as spam filter.
I’ve been using https://utteranc.es/, same concept but using Github issues as comment. This is interesting, especially if there’s a way to handle each user’s instances.