In case you have two-factor authentication enabled for your iCloud account, you will need to create a special password for Spamdrain in order to allow our servers to access your mail. Apple specific passwords allow you to sign in to your account securely when you use Spamdrain.
Please refer to the following step by step guide from Apple:
If you have two factor authentication disabled, please feel free to enable it and create a special password for Spamdrain to login. Newly created password should be entered in the password field at your Spamdrain's settings page.