We're sorry you're experiencing this problem. 😕 This can sometimes happen when there's an issue with your Polymail preferences.
Going in and removing your Polymail Preferences files will force Polymail to recreate it and should resolve the problem! 🎉
We'd recommend doing the following:
Close the Polymail macOS app
Open to Finder.app
Use the keyboard shortcut CMD + Shift + G
Type in "~/Library/Preferences/ctrlla.Polymail.plist"
Delete the files with the name "ctrlla.Polymail.plist,"" io.polymail.osx.helper.plist," and io.polymail.osx.plist
Restart Polymail macOS app
Not to worry! This only removes Polymail's Preferences file. 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 the Polymail Application Support files as well.