I had this same issue. I realized I was losing a sizable percentage of new users. So I attempted to manually activate their membership. But I got an error message. 

I figured the activation notices were going into their spam traps.

My goodness, thank God that UNA does not draw as many spammers as Dolphin did.

Please keep us informed about this matter. Me, I am positive that our other email notifications are being sent and received. It was just my manual approvals which always failed.

Oh by the way, what would you say if I created a new profile here: "Hodor Hodor Hodor." Would you be jealous of me?   :)