I'm assuming the message in the email has either been addressed or it is otherwise not needed - IE its not an error that needs looking after.
Rebooting wont fix the problem. Once the system comes back up, it will just try to send it again.
You could look in /var/spool/mqueue and try to identify the appropriate d* and q* (text) file pairs that contain that particular email and delete them.
Does this problem exist for just one account? multiple?
For the account in question, is it just this one email that keeps getting sent or is it for all emails from this account? - just some things to think about that may help to ultimately solve the problem.