4/30/2008 -- Postmark generation is a new feature that's only available in Outlook 2007. However, postmark validation is available in Outlook 2007, Windows Mail in Vista, Windows Live Mail and Exchange 2007.

Basically, Outlook 2007 messages can be stamped with an e-mail postmark that includes the time the message was sent and a list of the recipients. This adds a slight overhead to the message but it's hardly noticeable. The recipient's e-mail application will recognize the postmark and won't treat the message as junk mail.

The idea is that spammers will be discouraged from using postmarks because they send thousands of e-mail messages at a time, and adding postmarks will be too much overhead for them. However, an average user won't have to worry about this small overhead and can take advantage of the postmark feature.

There are some situations where messages aren't postmarked. For example, messages aren't postmarked if the recipient is in the sender's Global Address List. Also, before sending a message, the Junk E-mail Filter in Outlook 2007 evaluates the message's characteristics; if it determines that it's not likely to be considered a spam message by the recipient's spam filter, it won't postmark the message.

The postmark feature is enabled by default. If you want to turn it off, you can go to Tools, Options, Preferences tab, E-mail, Junk E-mail, and clear the check box that says, "When sending e-mail, Postmark the message to help e-mail clients distinguish regular e-mail from junk e-mail."