Just as it might have seemed that sendmail had all the features you could possibly want, not to mention lots more you didn't want, a new problem arrived on the mail scene. Out from the primordial scum emerged a new breed of cockroach. Previously passed by by evolution, this parasite emerged with an attitude problem. This, of course, is the spammer.
There was a time when all mailers on the Internet would accept mail from anywhere, and send it on anywhere else. Recently, ISP's, and other owners of mail hosts have spent a large amount of time and effort to remove this functionality, and to ensure that mail hosts will only relay for machines authorized to use that host. Spammers would (and will) use open mail hosts as injection points for vast volumes of bulk email, often crippling systems for real users, and polluting the mailboxes of the world. For a user to find spam mail in their inbox is annoying - for a mail system administrator to find that their host has been used as a spam relay is catastrophic. The system used as a relay can bog down under the load, and, even worse, the relay systems are often wrongly identified as the originators of the spam, asd so receive a vast number of bounces, and complaints. If they are unlucks, they may be placed on one of the Internet's blackhole lists, which can result in them losing mail connectivity to large amounts of the Internet.
Sendmail 8.9 has many new features, but the most important of them are there to prevent spammers from abusing mail hosts. Sendmail will no longer relay all mail by default, and it is recommended that administrators set up the access_db to control who has access to their host. Sendmail also now defaults to giving a temporary rejection for mail where the sending domain does not exist in the DNS. This feature helps to prevent the forgery of mail headers, by rejecting bad domains. These changes to default operations are the most significant difference that users will encounter while moving from sendmail version 8.8 to version 8.9, and are one of the most common causes for an upgraded sendmail to fail with an older configuration file.