Are those valid emails? Are you sure your account was not hijacked for spam?
If the account info is not in Outlook somewhere, there is no way it's capable of sending out the emails. Go to the Mail setup in control panel, make sure only your account is there. Also check any settings under "other user's folders" make sure nothing is there to open up another mailbox.
Is this on a corporate server or just webmail like Yahoo or something? If it's in a company, have your IT person(s) take a look at it.