For inspiration, here’s the text we use in our Drip post-cancellation email:

Subject: A quick question…

Hello,

I was hoping you could spare 15 seconds of your time and let me know why you decided to cancel your Drip account. Feel free to just hit reply and fire away.

Thanks in advance,

Rob
Founder, Drip

P.S. I’d really appreciate your reply – even if it’s just a few words letting me know why you decided to cancel.