This can be used for Thunderbird, the Mageplaza SMTP Magento2 plugin or any 3rd party Gmail app.
Login to Gmail, then go to "Manage your Google Account".
Then click "Security" or go straight to https://myaccount.google.com/security
2 Step Verification should be turned on already.
If it isn't then set it up!
Then go to "App Passwords"
where it says "Select App" - choose Mail
where it says "Select Device" - choose Other (Custom Name) and type "SLVUK Website".
then click Generate.
It will generate a one-off 16 digit code that can be used in your 3rd party Gmail SMTP password.
Login: the full gmail email address
Password: <the one given above>