Unspecified font used in BASE64 encoded mail

Hi, I am using Japanese language and eM client works fine with Japanese. While almost all mails are displayed correctly by my specified Japanese font through   Tools - Settings - Mail - Read, there are some mails ( google calendar invitation mails ) which use unspeficied font ( actually Chinese Font ). What are in common for them is values in content related headers as shown below:

Content-Type: text/plain; charset=UTF-8; format=flowed; delsp=yes
Content-Transfer-Encoding: base64

Any advice for forcing them to use my specified font would be appreciated.