Email Address Policy suffix not working

Hi
We have set up a second exchange server in our organisation for student email and have attempted to create a Email Address policy which appends student.domain.ac.uk to accounts. We have created a mail enabled Security Group labelled StudentEmail on the OU where the student accounts are created. However when we create a new mailbox for an account in that OU it appends it with the default domain.ac.uk suffix. I have followed the microsoft articles and the advice I have found on a few other sites but I can not get this part to work. Any help would be appreciated.

Thanks