Sender ID - technical definition

An e-mail authentication system from Microsoft that is based on Sender Policy Framework (SPF) records in the DNS system. Sender ID uses Microsoft's proprietary Purported Responsible Address (PRA) method for checking the headers within the message body. Sender ID also supports SPF Classic, which looks only at the envelope headers outside the message body; however, Sender ID generally implies the PRA method. See SPF and e-mail authentication.