Open-source it. If they're really abandoning the product but actually care about improving the status of e-mail clients they won't send you a C&D.
If they don't care about that as much as they care about preventing unauthorized use of their code, then everyone loses. But you can't know what'll happen unless you take action.
Agreed! Maybe it can take a life on it's own. No use shedding tears over what's done, but open sourcing it would at least mean there is a net gain for the world from it all.
I don't know but it wouldn't be unprecedented. For example, Google open-sourced etherpad when they bought the company (mainly for the talent rather than the product).
If they don't care about that as much as they care about preventing unauthorized use of their code, then everyone loses. But you can't know what'll happen unless you take action.