We're sorry to hear that you lost your device, and we hope you find it soon!
In the meantime, you can force a logout of your Polymail account on all of your devices so that you won't receive any new email (or be able to access existing mail) on your lost device. To do so:
You'll see a blue link to "Sign me out of Polymail on all devices." Click this:
You'll now be logged out of your Polymail account on any associated devices. Not to worry, though! Anywhere you re-login to Polymail, you'll be able to access your email again. 👍