Users not getting activation email
I was having the issue which many people have posted about - where simply no registration email is sent out, or if it is, only to addresses on the same domain.
For me the solution was simple. Install the "Configure SMTP" plugin by Scott Reilly and configure my SMTP mail server settings in this. This plugin is very old and this did concern me, but it seems to work very well. Hope this helps some others facing this very frustrating issue!