ADSP stands for Author Domain Signing Practices and is a valid DKIM Signature in which the domain name of the DKIM signing entity matches the domain in the author's address. Implementation is not wide spread: The OTA reports that they have been tracking 1,250 domains over the last 3 years (Fortune 500, Internet Retail 500, FDIC 100, Top 25 Social Sites and Top 50 Federal Sites) and only 1 domain (Paypal) has an ADSP record.

According to some experts, while signing with DKIM is probabaly a good thing for delivery, deploying ADSP isn't (see attached OI discussion thread. According to that discussion, ADSP will not improve delivery rates, and may actually cause delivery to get a little worse. Experts warn that before deploying ADSP you need to be entirely sure that you're routing all email via smarthosts, including all email that uses the same domain in the From:field (such as corporate email, customer support email, server monitoring, auto-responders, and any affiliate programs). Since you need to send from your smarthosts, any employee with laptops will need to VPN in to send corporate email.

A detailed discusion can be found here: ADSP USAGE