This is a very old post so I assume you have resolved it.
I recently had the same issue - worked fine on Win but not on Mac. It turned out my default mail client in Mac Mail was set to Google:
Mail->Preferences->Default email reader
I did not manually set that, but in some development work had experimented with sending a mail via google as opposed to desktop mail client - thats the only thing I can think that would have caused it.
Only discovered this by chance out of a discussion from a non-FMP user (after reinstalling FMP etc).