If you are referring to an Outlook.com email account it is probably because Microsoft are allowing averts to be attached to pay for the service.
If it is on the email programme and you are using a BT email account it may be that you have a "signature". You need to go into the email accounts settings on Outlook and find "signature" and delete it.
EDIT: Just a thought. Are you sure that it is not just you that is seeing the advert in your browser and when the email is sent the recipient does not see it.
