A few days ago I booked a flight with ThomsonFly, something I've done many times before. The confirmation email sent to my plusnet email address didn't arrive; nor did the subsequent copies they sent me. In desperation I set up a googlemail account and had them send the confirmation email there. Thankfully it arrived. There is nothing unusual about my plusnet account or email address. I have had problems in the past with plusnet rejecting emails from certain friends, but this strikes me as a serious problem for all plusnet users.
You've no spam filtering enabled for your domain. Is that something you've only recently changed? If you previously had the 'Discard Obvious Spam' option enabled then that could explain why the email went missing. Failing that, I would need to see the full headers of the message sent to the GMail mailbox to see if I can work out what's going on. If the IP of the sending MTA and the content of the message checks out though, there's not a great deal we can look at (without trawling through server logs, which we only keep for so long).
Bob Pullen Plusnet Product Team If I've been helpful then please give thanks ⤵