You are an email administrator for a large company. Recently a series of emails were sent from a user's email account that the user claims not to have sent. How should you proceed?
Ask the user to please stop sending the emails
Disable the user account, reset the password and run an anti-malware scan on the user's PC
Tighten firewall restrictions to block the SMTP port (25)
Disable the email and provide a new account to the user