Troubleshooting the Recipient Update Service

http://support.microsoft.com/?id=288807 also

the Exchange team Blog - there is a 4-part series on troubleshooting RUS

http://blogs.technet.com/exchange/archive/2004/07/07/175444.aspx
http://blogs.technet.com/exchange/archive/2004/07/15/184356.aspx
http://blogs.technet.com/exchange/archive/2004/07/22/191513.aspx
http://blogs.technet.com/exchange/archive/2004/07/27/198662.aspx

http://support.microsoft.com/kb/286356/EN-US/

http://www.windowsitpro.com/Articles/Print.cfm?ArticleID=37540

http://winzenz.blogspot.com/2005/10/outlook-autocomplete-and.html