I'm configuring a shared mail box with permission to send on behalf of, however, the email is not being sent from Outlook and the message is not being saved on the sent items from the shared mail box. Why?
My scenario is: Exchange Online (Only) + Outlook 2016 ProPlus
What do you mean by "the email is not being sent from Outlook"? As for copying messages sent as/sent on behalf of shared mailboxes, this can be configured via PowerShell and I believe is the default option now. Just in case, here's the cmdlet: