Self-activation email not working

I’m running EE 2.11.2. I have my member preferences set to Self-activation but members are not receiving the activation email when they register. I tested the Communicate tool, and emails are being sent successfully that way.

I have the reply email set up on my hosting server (Media Temple). I also made sure that I have a Webmaster name set up so it’s not blank.

The user is being successfully registered but they remain pending. Even I if I choose to send activation again, it still doesn’t work. Any ideas?

Jeremy