This is a mailbox permission issue. Here's the step by step procedure to assign the permission. For example. John King will give permission to Mary Queen to open his mailbox and send on behalf while he's out. Open Active Directory (AD) Go to the User OU, right click on John King properties Click Exchange Advanced, click Mailbox Rights. In Mailbox Rights, click Add Mary Queen, and give Permission Put check on Delete mailbox storage, Read permissions, Take ownership, and Full mailbox access. Click OK Click Exchange General Click Delivery Options In Send on behalf, click Add Mary Queen, then click OK We just completed giving the right permission to Mary Queen to access John King mailbox. Now let's add John King mailbox to Mary Queen outlook.