Você deve ser capaz de forçar o Outlook a exibir as mensagens como texto sem formatação por KB relevante . Verifique lá para o local de configuração e anote esta parte -
In the case where digital signatures are used, Outlook must display the e-mail message in the original format that is specified by the sender, or you must turn on the Read all digitally signed mail in plain text option. To turn on the Read all digitally signed mail in plain text option, click to select the Read all digitally signed mail in plain text check box under the Read all standard mail in plain text option.