Here's the suggestion: >Use <service>@<yourdomain> as your email address when signing up, and check the To header when receiving emails. The user of the webse

Here's the suggestion: >Use <service>@<yourdomain> as your email address when signing up, and check the To header when receiving emails. The user of the webservice specifies a unique email per webservice; knowledge of that unique email address serves as a hint that the email came from someone that has discovered that email address, i.e. the webservice itself.