Migrated mailbox

Yesterday, when I opened my mailbox, I noticed that it seemed to be corrupted. All folders I use to organise the vast flow of mails I receive every day had disappeared. After checking the university helpdesk, I learned the IT-department had started migrating the current student's mailboxes to the new server that evening. After further investigation of my 'corrupted' mailbox I found a mail (with no subject) which stated that my mailbox was migrated and provided the new settings to be used in my E-mail application.
I was a little relieved when I changed the settings and after connecting to the new server, I noticed that my folders appeared again. But not all. Two of them where still missing. When trying to create the folders in the new mailbox, I found out why they where missing: the folders contained a '@' in the foldername, and apparently this character was refused for foldernames in the new mailbox. This explains why these two folders weren't copied when my mailbox was migrated.
But one of those folders contained over a hundred E-mails I still needed. After copying the original mailfolder, which wasn't yet deleted from the old server, using another server I managed to restore them.

So now I have a new mailbox on a new server, which has a lot of advantages. The old server was getting slow, as it is quite some years of age and has to serve the mailboxes of over 27,000 students at my university. During last semester one had to be very patient when checking E-mail arround lunchtime.
Checking E-mails should be much faster now with the new server.
My mailbox is a lot bigger too and doesn't consume space in my networkdrive.

I'm very pleased to already have been migrated to the new server. But I'm wondering what criteria where used to select which student's accounts where to be migrated first.