Dear (former) colleague,

I'm sorry you felt that my arrival, coupled with some recent changes within your organization, were some kind of threat to you. I was more than willing to help you learn the ins-and-outs of a role you weren't familiar with, but with every offer of advice or help you chose to meet me with aggressively bitter resistance. I can't help that now.

I've just heard that you resigned today. I wanted to wish you the best of luck, but you've chosen to ignore e-mails coming from my address. That's fine. I'll just wish it to you here instead.

Regards,
The New Girl On The Scene (who now gets to try to figure out what your intentions were with this product without the benefit of an actual conversation or knowledge transfer with you)