Shared mailboxes are handy for a variety of functions that involve several users monitoring a mailbox.
However, it’s not uncommon for a user to need to reply to the message as the shared mailbox address.
This can be done in Exchange 2013 CU9+ and Office365, but it is NOT enabled by default on the shared mailbox.
To enable this feature, run the following:
set-mailbox SharedMailboxName -MessageCopyForSentAsEnabled $True
Assuming you’ve already added your user to the ‘Send As’ group on the shared mailbox, he or she will now be able to send emails as the shared mailbox and have them show up in the sent folder on the shared mailbox.
It should be noted that this same cmdlet cannot be used on a user mailbox. If you try, it will bark at you.