Is there a mechanism in place to authenticate an SMTP server as an authorized or legitimate sender of e-mail for a domain?

Two mechanisms used to check if a mail-sending machine is legitimate for a domain are Sender Policy Framework (SPF) and Sender-ID. SPF provides for using DNS records to define which machines can send mail for a domain. For a good overview, check out the OpenSPF site and the Wikipedia entry on Sender-ID. Sender-ID is a variation on the theme and is essentially a superset of SPF. Sender-ID utilizes the same type of DNS entries but uses them differently to determine whether mail is being sent legitimately. Publishing SPF records for the mail host puts you in a position where the receiver side can verify that the mail came from a legitimate mail sender for the domain. The DNS entries used by SPF and Sender-ID are TXT records that look like mail.yourdomain.com IN TXT “v=spf1 a all” . 

There is an online form wizard useful for creating initial SPF DNS entries along with links to FAQs and a list of common mistakes to avoid when creating your records. There are also links to SPF testing tools available there. SPF and Sender-ID are useful tools for providing other domains the ability to determine that your e-mail has been sent from a legitimate mail host. This is becoming more important as more mail services (such as Gmail) implement SPF checks on incoming mail.