...
Spamass-milter 0.2.0 includes a new feature that constructs a pseudo - Received: header to provide SpamAssassin with additional useful data. Since incoming email is passed to the milter prior to SendMail adding its Received: headers, spamass-milter constructs a Received: header itself to pass to SpamAssassin so that SpamAssassin can take the information into consideration. This header is only used by SpamAssassin as spamass-milter does not send this header back to Sendmail. Sendmail continues to add its own header as it normally does. The following line is the code in spamass-milter that send the pseudo header to Spamassassin:
...