Like many other users, I was having problems not receiving the Account activation email for user.
It took quite a while before I realised that the emails were being sent, but ending up in my junk/spam folder instead of my inbox.
It took even longer to work out why. The fix is to change the subject line from: Please validate your (site name) account to: Your (site name) account. I suspect the original wording of the subject line triggers the spam filter.
Hope this helps someone. Gramps.