After migrating to Exchange 2000 Server, we assigned delegate access to several executive-level mailboxes. Although the delegates can access the mailboxes from Outlook, they can't make changes (including deleting messages) to the executives' mailboxes from Outlook Web Access (OWA). What's going on?
OWA 2000 doesn't support full delegate access. Instead, delegates receive read-only access. If you need delegates to have complete read/write access to mailboxes from OWA, you must grant full mailbox permissions to the delegate accounts. To do so, use the Microsoft Management Console (MMC) Active Directory Users and Computers snap-in to open the Properties dialog box for each mailbox for which you want to grant delegate access, then select the Exchange Advanced tab (you'll only be able to view this tab if you select the View, Advanced Features menu item). Click Mailbox Rights, then add the delegate user and select the Allow check box to grant that user full access.