Hi there,
You could troubleshoot what will happen on a test account. Basically, that user's existing email accounts will exist but they won't be able to send or receive emails due to being over disk quota. The email accounts won't disappear. Next, the user won't be able to use the account because they will be over disk quota as well, so no contents could be uploaded. The site would continue working for the existing files.
Again, you can always test the questions being asked to see the results by setting low levels such as 10MB and 1 email account then putting a test account into that package.
-Lochie