I am the hosting service for my site. I run a web hosting business.
The SMTP settings are fine. I am using my domain SMTP settings with the correct email, port, server name, password. Works fine sending a test from the admin control panel but activation emails are not going through. They are also not ending up in people's spam folders.
I can send an email using my mail client and the same email address used by the forum, to any other email account and it arrives in seconds. Yet the same thing does not work from the forum. However as mentioned, this only affects around 50% of signups. Some do receive the activation emails.
It is a complete mystery. And it also seems peculiar to PhpBB because I also run other forums and activation emails from them always go through.
It is incredibly frustrating and means I have to manually approve all registrations, which on a board like mine is a full-time job!