BuddyPress.org

Check with your host, to make sure you’re not exceeding your email limit, or that you’re not getting throttled for activity or user abuse. I suggest this first because you seem to have an inconsistent pattern of users receiving and not receiving. Also, I would do some test sign-ups using different emails to see if there is a pattern there, and then I would start looking for plugin conflicts. Please let us know what you determine, it would be helpful to know