Welcome to Forum!! Suggests to go through below links carefully http://sql-server-performance.com/Community/forums/t/2687.aspx & http://sql-server-performance.com/Community/blogs/othersqlserver/archive/2007/09/12/database-mirroring-check-the-network-address-name-and-that-the-ports-for-the-local-and...

Also ensure that firewall rules are perperly set, refer http://msdn2.microsoft.com/en-us/library/aa337361.aspx and http://www.sqlteam.com/forums/topic.asp?TOPIC_ID=67626