Depending on your provider, recipients mail hosts might be blocking your mail sends due to spam blocking.
This happened to me. Apparently a certain user of mine never received the email because his mail host was blocking the ip range hosting my server because they subscribed to an anti-spam service of sorts. The service had detected that my web hosting provider had a client that was spamming people so the service blocked the provider's whole IP range.
So, any emails sent to this user would never be received.
Fortunately for me, my hosting provider sends me an email automatically whenever there are any undeliverables or errors and explains the problem. I reported it to my provider, they banned the spammer account and got in touch with the anti-spam service about re-allowing their IP range to send mail to their subscribers.
If it wasn't for my provider sending me that automated email, there would be no way for me to have figured out why the user wasn't receiving the activation emails.
No idea if that's what's happening with you, but it's a possibility.
On a much simpler note, be sure you can actually send email through your provider successfully. Try sending yourself a form email through the email user function of the forum.