CaptchaMail's email forwarding service is the ultimate way to stop unwanted automated messages. Users provide two email addresses: one private and one public. Emails sent to the public address are forwarded to the private address, but only after the sender solves a captcha.
CaptchaMail requires access to the public email account in order to read and send emails. We recommend using an account specifically for this purpose, and users may choose to supply CaptchaMail with a revokable, temporary, and/or scoped "app password" instead of the account's regular password. OAuth is not supported, only "basic" auth. The account must support IMAP and SMTP, and for SMTP, STARTTLS is required.
Pricing is simple: 2USD/year.