Why do incoming email messages that are signed include an smime.p7m instead of stating that the message was signed?

I have a person outside of my organization that I'm working with that sent me a signed email last week. When I looked at it then, the email message in Outlook for Mac 2011 stated that it had been signed by her and it gave me the "Details" drop down that allowed me to view the certificates or add them to the Contact entry.

Now, when I go back and look at the email today, it's no longer giving me the signed message banner and is instead showing an smime.p7m attachment. I also received a new email today from her that was signed and it also has the attachment rather than the banner. What could have changed that would cause this to happen?