Mailtips will be really useful because they integrate with Exchange Server 2010 and this new capability can save a lot of time of our end-users. This feature can be used for email messages, meeting requests, sharing messages, and task requests.
How often have you sent a message to someone only to receive an Out of Office message in reply a moment later? Or (like me) you might have sent a message to a Contact Group, only to find out too late that group contained hundreds of people - when it happened to me I surely wished I had known in advance.
MailTips make these email "accidents" a thing of the past! As you start adopting Outlook 2010 and Exchange 2010 in your organization, we believe that MailTips will help make you feel more in control of your mailbox and more confident about the emails you send.
MailTips offer Outlook users real time information about the message they're composing and the recipients they intend to reach with it. You will immediately know information such as if a recipient is out of the office, if they're outside of your organization, or how many people are in a conversation group.
Read more at source: http://blogs.msdn.com/outlook/archive/2009/08/12/introducing-mailtips.aspx