McAfee has imbedded detection of DRM (Digital Rights Management) so if the MP3 has DRM applied it may be blocked.
I'm not going to get into the whole DRM concept and why it's terrible but sufice to say that DRM allows the "owner" of the music to control it's distribution. You aren't the "owner". You may have bought rights to listen to it but you don't own it (unless you created your own MP3 with your garage band...
). McAfee's software "helps" the owner control unauthorized distrbution - i.e. people e-mailing copies to their friends.
Your options are:
a.) Don't send it.
b.) Find an application to remove the DRM flags on the MP3.
c.) Turn off McAfee when you send the e-mail.