I recently built a Drupal 7 based website with the possibility for the visitors to register / apply for an account to view certain protected pages otherwise hidden from anonymous users.
I'm using the default process:
User applies for an account -> User (should) receive the "Welcome (approval required)" email -> Main site admin approves account -> User (should) receive the "Account approved" email containing the account details + the one time login to setup a password.
Everything is working as it should except the email bits. This is totally inconsistent and certain users receive the welcome emails and some users don't.
It doesn't matter what email they use: gmail, yahoo, hotmail, custom domains. For example I receive the welcome emails on my gmail account but not on an email address using a custom domain (email hosted at hostgator).
Furthermore, I've tested this with at least a dozen other emails, and I can't find a pattern.
Another important thing to mention is that the website is hosted on a Rackspace dedicated cloud server but the domain is only pointed to the server. The email records and NS records are on a different server. I only host the website, nothing else.
Could this be causing the issues? It's a domain trust / key issues perhaps? Or something to do with strict spam filters?
I would really appreciate some help with this.
Edit: I just want to add the fact that I've added a few triggers too (using Rules) to replicate the email sending process but it's the same.