After numerous requests we have added the feature to allow the SUBJECT line of the email SMS message to be set as the content. Normally you set the body of the email but this leads to some issues where when not formatted correctly, your email signature can be added. Also some systems may define the subject line as the content you require on your SMS. When using the subject line the entire body of the email is ignored.
By default when an Email SMS authorised address is set up, the body is used. To change it, simply choose Email Subject from the Message Content drop down in the settings for that address.