If you are not up to speed on Mailtips I suggest you reading this article by the MS Exchange Team:

http://msexchangeteam.com/archive/2009/04/28/451193.aspx

To enable Mail Tips you need to use powershell with the Set-OrganizationConfig cmdlet. There are various components of mailtips you can enable or disable.

- MailTipsAllTipsEnabled. Controls whether MailTips are enabled. The defaulting is $Right.

- MailTipsExternalRecipientTipsEnabled. Controls whether MailTips for external recipients are enabled. The defaulting is $Fake. External recipients are determined by reference to the accepted domains list. Any domain in this list is deemed internal; any other domain is deemed external.

- MailTipsGroupMetricsEnabled. Controls whether MailTips that depend on assemble sizes are enabled. The defaulting is $Right.

- MailTipsLargeAudienceThreshold. Controls the threshold for the number of recipients on a message before MailTips flags it as large. The defaulting value is 25. This value is probably too low for large organizations, where huge distribution groups are common. In this scenario, it makes sense to increase the value to 50 to stop MailTips from nagging users for no excellent reason.

- MailTipsMailboxSourcedTipsEnabled. Controls whether MailTips that depend on mailbox data such as out-of-office notices are enabled. The defaulting is $Right

Note: If all are enabled, it will increase load on your client access servers by 5%.

Check it out:Clint Boessen’s Blog