There are several different things that can occur, but you have few choices for your response/action.
It is difficult in outlook/exchange to log on to other accounts unless you are either allowed access in active directory or exchange. If somebody has your network logon, change. it. If they then try and log on as you, your account will lock out if your system does this and somebody can track the bad logon to a remote machine.
If it is somebody in IT or smebody has rights to your mailbox, that is another issue and you should discuss with IT/exchange administrators.
Totally concur the first answer. Access to your mailbox (possibly seen as an added mailbox to the other “user”) would have been applied/configured at Exchange System Manager level meaning IT Admin or someone with similar Administrator access rightsto Exchange server has specifically given permission.