I would talk to a lawyer before doing this... see: http://en.wikipedia.org/wiki/Tortious_interference

In what way would telling the truth in a private email constitute tortious interference?

Given the US, talking to a lawyer first is pretty much invariably the smarter thing to do. Even without going this far, you can make statements that then negatively impact your bargaining position later on.

The "truth" is only what can be proved in court. Lots of folks have been tripped up by that before... so be careful.

In this specific case the legal is likely very low. It's a startup and it's unlikely they have money to fund a lawsuit. Plus there's that whole "he's from another country" bit which would make collecting on a judgment pretty damn hard.

I'm not a lawyer, but my understanding is that the OP is in a foreign country and his work arrangement was one of remote work. So how would US tort law be enforced?

(That is not to suggest that talking to a lawyer first in these types of matters is a bad idea, of course.)

I don't see how this applies.

Assuming that the OP has a reasonably standard employment agreement, he may well be breaking it by contacting their investors (there is the tort). Assuming that he doesn't want the job back, why would he contact them other than to disrupt the relationship between YC and his former employer? IANAL, but the basic elements seem like they are there and I wouldn't even consider doing something like this without getting legal advice first.

"why would he contact them other than to disrupt the relationship between YC and his former employer"

He might want YC to instruct other YC-backed companies to avoid this behavior in future, sparing other people this problem.