This is because the signature contains hidden formatting codes as it was copied from Outlook or a Microsoft Word document.
To reset the signature you can do the following
Copy your signature from Outlook to a plain text file in Notepad
Copy this plain text signature from Notepad to your CRM Mail signature (User ; Profile ; Mail)
You will need to re-insert your logo using the tools within Access Recruitment CRM
Re-insert the merge field in your mailshot
Run a dummy/test run to see if the problem is corrected.
You can then use the formatting tools within Access Recruitment CRM to change the layout of your signature, insert hyperlinks etc.