I would question the wisdom of providing lifelong e-mail to students without making it obvious who is a student, alum, or staff. The schools that I know of do it like this:
user@staff.university.com
user@student.university.com
user@alum.university.com
That makes management much easier. However, you said that was not an option, so you will need to do things a little differently.
Here is how I would handle it.
I would have all mail for @university.edu routed to the outside provider. I would either have the outside vendor setup mail forwards for staff addresses (optimal), or setup accounts for staff and set up forwards from inside those accounts and have that mail forwarded to the GroupWise system.
Mail from staff to staff would only traverse the GroupWise system all other incoming mail would be routed through the outside vendor to GroupWise, but that could be setup to be virtually invisible (only seen in the headers).
Then you are only left with staff having different setups and webmail access.
As stated above I don't find this an ideal setup, but it should work. I hope you find this helpful.
The Student may or may not see the outsourced company depending on how you handle DNS, Web access . . . But with this setup and your outside providers help you can hide it from students, but not from staff.
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And here I thought I was just about the only remaining GroupWise user.