If you're noticing the the Polymail app is becoming unresponsive randomly and you're having to Force Quit the application, it may be the result of something going wrong in your Polymail Application Support Folder.
By removing the Polymail Application Support folder, you'll be able to force the Polymail App to recreate it and resolve anything that may be going wrong.
To start a new Polymail Application Support folder, we'd recommend you follow these steps:
Close the Polymail app
Delete the Polymail app
Opening to Finder.app
Using the shortcut CMD + Shift + G
Type in ~/Library/Application Support
Delete the Polymail folder in the Application Support Folder
Reinstall Polymail by downloading it from here.
Not to worry! This only removes Polymail's Application Support. It doesn't effect any other application or any files related to your macOS.
If you're still experiencing issues, it may be worth trying to remove Polymail's Preferences files as well.