Delete unneeded mail
Keep it if you need it, delete it if you don't. Review mail in your inbox, sent mail, and cabinet for items that are no longer needed. Start fresh in Office 365 by only taking what you need. This includes mail in your "live" mailbox as well as your archive. View some cleanup tips...
Clean your address books
Remove duplicate and inaccurate contacts. Consolidate address books when possible.
Contact the IT Services Help Desk if you are in one or more of these special situations.
Search for and remove messages larger than 25MB
Office 365 does not allow for messages over 25MB in size. Before migration, you will need to locate these messages, save the files, and then delete the message. Use these instructions to make this task simple.