Shared Mailbox delegate - block delegate from sending emails

[Marvin Oco]

Scenario:

1 regular mailbox for user@contoso.com

1 shared  mailbox shared@contoso.com

user have send on behalf rights to shared  mailbox shared@contoso.com

requirement is to block user@contoso.com from sending emails and force him to send mails using shared  mailbox shared@contoso.com

is this possible?

[#2]

You mean prevent him from sending messages with his own account? No way to do that I'm afraid. The only thing you can do is create a transport rule that blocks the messages after they are sent via the client.

[#3]

@Marvin Oco 

you can do it using third-party tools/services. 

Have a look at SmartReply cloud service.  It seems that "Uncoditional address rewriting rule" will fit your requirements.

If you run only Outlook for Windows, take a look at SetFrom add-in.