Since emails are not easily reassigned, they can only get a new handle. Two mechanism to get there:
Concede that "admin" is a terribly ambiguous term because every "task" implies the existence of "administrative task concerns".
Then ask the newly formed department to do the same, so that well beyond email handles they will self-brand/sign as something more specific than "admin". If they do things specific to the office, but are more than the janitor, maybe plain [email protected] will do the trick. In the future, insist on more clearly descriptive namespaces for every newly added name.
Use the opportunity to prepare departments that are expected to grow into managing some of their own tech for stronger namespace separation, e.g. [email protected].
I recommend against exclusively using the second one. Guess what happens if a location-specific department shares a name with a company-level department that eventually forms a location-specific unit. I have done this in a stupid way before and can thus attest that anything that does not minimize potential for future name conflicts and confusion will require disruptive change later on.
Also specifically in email context, you want neither namespace be a prefix of the other, because that will limit the flexibility of sub-addressing suffixes, which are a nice way to allow departments to smoothly adapt to increasing complexity/size/volume. So do not assume that you have found a great solution for the first one when you have merely come up with different abbreviations / extended spellings ("admin" vs "administrative").
