In order to ensure UF email continues to work quickly and efficiently, there are set retention periods on certain mail folders.
These folders include the Inbox (including any subfolders), Sent Items, Deleted Items, and Junk E-mail folders.
Mail in the Inbox (and subfolders) and Sent Items folder will be retained for 3 years and will then automatically be moved to the Deleted Items folder. These messages will retain the same folder structure, appearing as subfolders of the Deleted Items folder.
Mail in the Deleted Items folder will be retained for 30 days. After the 30 day period there is an additional 7 days available where mail can still be restored. See How to Restore Deleted E-mail
Junk E-mail is only retained for 14 days and does not get moved to the Deleted Items folder.
Any important messages that should be kept for a longer period should be moved to a top-level folder of the mailbox. Information on how to properly set up folders for retaining important mail can be found here,