Did you mean mail role as of what are the responsibilities for the email servers or did you mean the main roles as of what does an IT manager do?
For the email, you’d be in charge of it entirely for your unit.
For the main responsibilities, it depends if you’re part of a corporation or not? If so, some functions might not depend on the unit IT head directly. For instance, the emails servers might be located in the headquarters and so general management would be global. However, the unit IT head would be in charge of the local team that creates local accounts.
IT Head is responsible for for all IT related functions, he should be cost concius, must be leader in project management