By default, custom MailTips aren't displayed if the sender isn't allowed to send to that recipient. In that case, the Restricted Recipient MailTip is displayed. However, you can change this configuration and have the custom MailTip also display. For more information about configuring custom MailTips, see http://technet.microsoft.com/en-us/library/dd638199(v=exchg.141).aspx
However this link does not provide any information on how to proceed. Can someone please reach out to me if they have any info on how to display a coustom mailTip if the sender isn't allowed to send to that recipient.