Question : Exchange 2003 and 2010 coexistent - Outlook in 2010 cannot email to 2003

I have setup Exchange 2003 and exchange 2010 in the same domain and same network.  Outlook users in the Exchange 2010 server cannot send email to users in Exchange 2003.  From 2003 to 2010 is fine.  Using Outlook Web App in the 2010 server I can send to both.  I get a bounce back when I use Outlook from 2010 to send email to users in 2003 server.  I can send to Internet and Internet can send to both 2003 and 2010. The bounce back from Outlook in 2010 to 2003 is "There's a problem with the recipient's mailbox. Please try resending this message. If the problem continues, please contact your helpdesk" "Generating server: exchange2003.domain.com".  In short, everything works except sending email with Outlook 2007 in Exchange 2010 to users in Exchange 2003.  Any ideas?

Answer : Exchange 2003 and 2010 coexistent - Outlook in 2010 cannot email to 2003

After fustrating for a couple of day without a solution, I've decided the call the MS tech support guys.  They found event error 9667 in the Exchange 2003 server.  It's not even related to the OAB.
Failed to create a new named property for database "First Storage Group\Mailbox Store (EXCH2003)" because the number of named properties reached the quota limit (8192).
http://support.microsoft.com/kb/820379/en-us fixed it.
Thank you all for the effort.