====== Create a shared mailbox in Exchange ======
<code>New-Mailbox -Shared -Name "Internal IT" -DisplayName "Internal IT" -Alias InternalIT</code>

Or convert existing user mailbox to shared mailbox
<code>Set-Mailbox internalit@example.com -Type shared</code>
<code>
add-mailboxpermission -identity internalit@example.com -user EXAMPLEinternalit -accessrights FullAccess
add-adpermission -identity internalit@example.com -user EXAMPLEinternalit -extendedrights Send-as
</code>