Have you ever sent a really well thought out, important email, only to find out (through an automatic response) that the recipient is on vacation for two weeks? For most of us, this means either waiting two weeks for a response or emailing an alternate contact. For the recipient, it means dealing with tons of messages when returning from vacation.
This scenario is one of the many reasons, the MailTips in Exchange Server 2010 developed. MailTips are there to give you information about your message and its recipients before you hit the send button. For instance, this MailTip will show you automatic replies for recipients of your message:
When you know that someone is on vacation before you send them a message, you won't waste your time writing it, and they won't have to read as many messages when they get back.