Email servers, whether they are owned and operated by your corporate office or if they are Yahoo or MSN, are bound by rules and algorithms that determine the validity of the emails they receive. Obviously a great focus in recent years has been on creating rules by which to sort out the unwanted emails. By checking the actual behind-the-scenes "envelope" information of an incoming email, the receiving server decides if the sender if valid, forged, or possible SPAM - and all of this is automated.
Improving the success of your email deliveries requires an understanding of email authentication, and the main factor in this equation is your own email domain's SPF (Sender Policy Framework) record. Read about SPF at www.openspf.org and follow the wizard under the heading Deploying SPF.
To check your record, go to to a helpful page at the Microsoft site: http://www.microsoft.com/mscorp/safety/content/technologies/senderid/wizard/
and enter your domain name in the field on the right side of the screen. This will bring up your SPF record if it exists.
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