Microsoft Exchange Server 2010 analyzes the message, including the list of recipients to which it's addressed, and if it detects a potential problem, it notifies the user with Mail Tips prior to sending the message.
With the help of the information provided by Mail Tips, senders can adjust the message they're composing to avoid undesirable situations or non-delivery reports (NDRs).
The Mailbox Full Mail Tip is displayed if the sender adds a recipient whose mailbox is full and your organization has implemented a Prohibit Receive restriction for mailboxes over a specified size.
By default, custom Mail Tips aren't displayed if the sender isn't allowed to send to that recipient.
In that case, the Restricted Recipient Mail Tip is displayed.
Have you tired deleting and recreating a users ost?
Mail Tips are informative messages displayed to users while they're composing a message.
Last Year I migrated from SBS 2003/Exchange 2003 to 2008R2/Exchange2010.