In code alter user registration and override approval to no approval required In my module I want to override the approval for certain users so that they are immediately approved and get the corresponding email. 

The module looks at the subdomain to decide if approval is required. If it is, then the register-with-approve email should be sent. If no approval is required, the account should be activated and the no-approval-required email should be sent.

I have looked for hooks that I could use for this, but haven't got anything working yet. I'm using Drupal 7.
I suggest you to code this directly in your module :

use _user_mail_notify()