The error 535, '5.7.8 Username and Password not accepted' means that Gmail isn't accepting your login info.

My suggestion:

1. Make sure you've turned on 2-Step Verification in your Google account.

2. Go to your Google account's Security settings and create an App Password — you’ll use this instead of your regular password.

3. Please double-check that your EMAIL_PASSWORD environment variable has that app password set.