Office 365 (aka UTmail+) is U of T's collaboration suite. It encompasses email, calendar, Skype for Business. Please find set up Guides for various platforms below.
Check your "Inbox" first
Sort it by size and delete older emails and emails you no longer need
Check your "Sent" folder
Check the size and delete the older ones
Empty your "Trash" with the deleted items
Archive emails on your computer
Make sure the sender is using your @utoronto.ca email account for invitations and not your @utsc.utoronto.ca If the calendar invite is in your @utsc.utoronto.ca inbox, move it to your @utoronto.ca inbox and accept