[mmorse]

You may also find these account status modes useful:

Closed -When a mailbox status is closed, the login is disabled, and messages are bounced. This status is used to soft-delete an account before deleting it from the server.

OR

Locked - When a mailbox status is locked, the user cannot log in, but mail is still delivered to the account. The locked status can be set, if you suspect that a mail account has been hacked or is being used in an unauthorized manner.

Though you don't need too, you might use locked if your planning on lifetime email forwarding or something... (Which is something that more and more schools are starting, or at a minimum forward for at least a year etc.)

Are these just prior enrolled students/recent grads etc? You may take a policy something like:
-Notify them that you're going to be deleting their old mail & that new mail will be around for only 7 days or something. At the same time you could give them directions for setting up forwarding (either within the web-client or a portal). It may be beneficial to provide directions on copying their mail via IMAP/POP.
-Set the spam/junk & trash lifetimes to 1 day
-Wait a 'few days'
-Set email message lifetime 7 days
-After another month or so lock all the accounts & switch to email forwarding only configurable in a portal or by request.
-After a year or whatever (if you're not a 'lifetime' kinda school) then delete the accounts