Maybe your have MAIL_DOMAIN set to mail.domain.com and your forum domain is domain.com so the MX record was what made everything work and you shouldn’t have removed it after all. And it gradually went out of fashion as it propagated. So you need to put it back. Sorry if I had a hand in messing it up for you. And if you need that MX record but can’t have it and use the free email service, then you’re back at square one there too.
I’ve deleted my previous message in case it leads to any confusion.