A. It's not currently possible to perform Send As operations using OWA (as of Exchange Server 2007). If you have full mailbox permissions for another user you can switch to that user's mailbox and send mail. To switch to another user's mailbox, you have two options with OWA 2007.
1. You can add the email address of the user you wish to send on behalf of to the end of the OWA string (e.g., https://email@example.com).
2. You can select the drop-down box next to your name in the main menu bar, then enter the name of another mailbox to open.