When playing with limits you have to be careful as Enhance sending limit pet IP will count Server IP sending from the hosting server.
For example: if you have 100 domains and all send PHP emails, the respective limit set in enhance will count the server IP as a sender, and even if every domain sends 100 emails each, the limit is actually 100 emails x 100 domains from that IP.
This can create issues if you set up a low limit thinking that you limit an external IP (considering it is one user) and put a lower limit, like 200 emails/hour. This is not the case.
There is a lot of work to be done in the email area 🙂 with time we might get the desired functionality.