AT&T Postmaster Website
When AT&T customers authenticate and send email messages through Yahoo! or other servers, a digital (DKIM) signature is added to the message to associate it with an AT&T domain.
Authenticated messages sent through Yahoo! servers will contain "s=s1024" in the DKIM signature, while those sent by our customers through AT&T servers will contain "s=dkim01."
All authenticated messages will contain a "d=" value in the DKIM signature for one of the following domains: