Rich has two domains registered at Hover. One personal and one business. He then uses it with Gmail and his Apple Mail client. But recently, several emails have been lost in transit. Some are personal, some are business. Leo says that there are "black holes" in the system, due to spam filtering. It's possible that is what is happening between Gmail and Hover. If the carrier judges that the sending IP address may be a spammer, then the email will simply vanish. Or it could be intercepted by the spam filters. Gmail is very aggressive about fraudulent emails. So it may have judged that any emails going through Hover may be false or spammed. So he'll want to check that out. He can avoid Hover email altogether by simply making the forward through the MX record.
From Retcon5 in the chatroom: Check out MXToolbox.com. It'll look to see if your MX record is properly set up.