Opt-In Mail is a small application to send e-mail. You don't need to use your provider's SMTP server because Opt-In Mail sends e-mail directly.
The key feature of the program is an ability to sign your message using Yahoo! DomainKeys technology. Opt-In Mail adds a header named "DomainKey-Signature" that contains a digital signature of the contents of the e-mail. The DomainKeys-enabled receiving email system Extract this signature and claimed From: domain from the email headers, fetches the public key from DNS for this domain and verifies that the signature was generated by the matching private key.
