The confidential contents of your inbox could be sent to other people. Think of our computer system as an apartment building. If someone gets a key to the front door, it is much easier for them to break into the apartments inside the building. Your userid and password are like that front door key - they give you access to the Queen's network. If someone is able to use your account, it is much easier for them to break into other accounts and systems within Queen's.
Users are ultimately responsible for any and all use of their computing accounts.