After I set MailQueue = enabled, #1 (sure) I did not receive more of these error messages and #2 (pretty sure) I received emails from my forum, as much as I remember. I am not 100% certain as lot of things happened ( I moved also to a new server during xf1-xf2 upgrade ).
It's worth noting that if you disable the queue, that will stop processing anything that's in the queue already. If you re-enable it, those mails will still potentially be sent. (Thus if you disable the queue, you should empty it manually if necessary, though really this is more of a potential debugging thing. Arguably, we should just remove the option.)