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.