I notice there is a signature separator -- shown. Therefore the gibberish is something derived from a signature which is being auto added. Do you use a signature html file or have you created and added a signature in the Account Settings for the account ? Do you use a template which includes a signature? This may a template created using an addon extension. Right click on mail account in Folder pane and select 'Settings' If anything is in the 'Signature Text' text box then clear that text box and click on 'OK'.