Setting Message Class via Content-Type for Faxes

Hello,

I am trying to set the message class for inbound email messages (with a fax attachment) from the internet.  The class that I am trying to set is IPM.Note.Microsoft.Fax.CA.  Exchange 2013 SP1 is being used.

I have a transport rule set up which evaluates criteria on the incoming message, and then sets the Content-Class header to fax-ca.  Unfortunately, even though the content-class header is being set (I can see this is the headers when the message gets to Outlook), the message class is not changing and the message still has the IPM.Note class.

On the other hand, if the transport rule is modified to set the Content-Class to voice-ca, the message class changes to IPM.Note.Microsoft.Voicemail.UM.CA.

So it seems that it works for voice-ca, but not fax-ca.  Is there an Exchange setting that I have missed?  Is there a reason why this wouldn't work for fax?

Thanks.