Sparrow for Windows was just a few weeks away
Something doesn't make sense.  From the ambiguity of your description, it sounds as though you own the code, and that you had no formal business relationship with Sparrow, and that you accepted no money from Sparrow (it is really strange that Sparrow would even encourage you into such a position, and that you would put yourself there --- you left them in the situation where they could turn your work down, and develop their own Windows client).  If that is all true, then the code is yours, and you can turn it into a Sparrow replacement.  But you are giving off a sense of finality, as if what you wrote is useless.  This confuses me.   Think like an entrepreneur, not an employee.  If you have roadblocks to selling the e-mail client yourself, overcome them (e.g., if you are worried about UI copyright, then change your UI, and then sell it).  Once you earn money from it, move to writing a Mac version, an iOS version, etc.