There are two ways to give other users the ability to "send as" another user, mailbox, or public folder address when you use Microsoft Exchange server. The first is using the Delegates feature on the Tools, Options menu. This is configurable in Microsoft Outlook by any user and adds "on behalf of" to the From field: From: Bill on behalf of Mary.

If this is unacceptable, the administrator can give the account "Send As" permissions.

Once Send as permission has been granted, users will type the mailbox name or smtp address in the From field when composing a message to send it as the other user or public folder. Their own email address will not be exposed on the message header, however all sent messages will be stored in their own sent folder.

If you want avoid typing the From address, see the Tools listed below. Microsoft Outlook 2010 allows you to have multiple Microsoft Exchange server accounts in one profile and replies from the correct account.

Tools

Configure Mailboxes

1. Open Active Directory Users and Computers on the Exchange Server.
2. Go to the View menu and choose Advanced.
3. Right click on the mailbox you need to configure Send As on. Choose Properties.
4. Click the Security tab. (This is visible only if you selected Advanced in Step 2).
5. Click Add and select the users name that you want to give Send As permission to.
   
6. Find Send As in the Permissions list and check it.
7. Click Apply and close the dialog.

Configure Public Folders

1. Open Exchange System Manager.
2. Browse to the Public Folder tree.
3. Right click on the public folder and choose Properties.
4. Click on the Permissions tab.
5. Click on Directory Rights button.
6. Click Add and select the users name that you want to give Send As permission to.
7. Find Send As in the Permissions list and check it.
8. Click Apply and close the dialog.