send as permission in Exchange 2000

I need to access two mailboxes (A and B) in my outlook 2003. I’m using A account, so I add B mailbox by referring to http://www.msexchange.org/tutorials/MF006.html. In order to make A has the Send As permission in B mailbox and when sends a message through B mailbox, the From field shows the B as the sender,I configured by referring to http://articles.techrepublic.com.com/5100-6345_11-5035124.html.

After that I forward B’s email, I got the undeliverable message: “You do not have permission to send to this recipient. For assistance, contact your system administrator.”

Eventhough I grant A full permission in B’s mailbox, I still get the same error.

Anyone can help me regarding to this issue? Any help would be appreciated.