Google Apps Deployment Quick Start (Apps100)

An SPF (Sender Policy Framework) record is a type of DNS record that identifies which mail servers are permitted to send email  on behalf of your domain. The purpose of an SPF record is to prevent spammers from sending messages with forged From addresses at your domain. Recipients can refer to the SPF record to determine whether a message purporting to be from your domain comes from an authorized mail server. 

Another supported standard is DKIM (DomainKeys Identified Mails) which adds a digital signature to the header of mail messages sent from your domain. Recipients can check the domain signature to verify that the message really comes from your domain and that it has not been changed along the way. 

Note:  If you use an outbound mail gateway that modifies outgoing messages, such as when Postini adds a compliance footer, the change invalidates the DKIM signature. You need to prevent the gateway server from modifying messages or turn off DKIM authentication. SPF is the better choice in this situation.

Google Apps supports and authenticates both methods. Both methods have you add a TXT record for you domain(s).

Note: It can take 12-24 hours for your MX records to propagate through the Internet. 

Reference Material