However, a lot of people are a little freaked out by rodents and such or otherwise have bad associations with them (Plague carriers, uninvited "house guests" eating through walls and leaving rodent feces all over, etc.) and may find a highly realistic rodent type thing staring back at them from their desktop and/or taskbar to be a little off-putting.
So, my suggestion isn't to necessarily ditch the idea of a using a fossa as a logo, but rather to consider anthropomorphizing him. You know, like Mickey Mouse or the GIMP (Photoshop type software) rodent. Friendly and cartoonish and/or with some human-type attributes, but still a fossa.

I know MoonChild and some others here are into creating artwork featuring animals who have some human attributes, so maybe this would be something people would be eager to volunteer for if the devs are alright with considering a change?
It's just a thought.
I think it'd make the software a stronger competitor for some Thunderbird users looking for a similar program with Thunderbird's future now in doubt.