Earlier quoted context omitted.
Why would you expect any institution to support all email addresses of their ex-employees ad infinitum? This would be a security nightmare for them. It is pretty normal for universities to have some sort of identity managmemt system that automatically provisions emails when you are employed there and deprovision them once you are gone.
Why not have a system where students and staff have actual email inboxes but alumni have their email forwarded? Most universities use a portal of some sort for easy access to personal information and preferences anyway, so it shouldn't be too difficult to limit access for alumni to only allow them to change a few personal details like name / address / phone number and the like, plus email forwarding settings. I think…
Obviously our university isn't gonna make a 10k€/month contract just because some prof wants their mail forwarded to gmail. Especially not if they are not working here anymore.