CIS must grant you Send As permissions on the shared mailbox before you can send a message from it.

Open a new email message.
Select the down arrow in the From button of the email. If you do not see the From field, go to Options and click the From button.
If the address from which you wish to send is displayed, select it and proceed to step 6. Otherwise proceed to step 4.
Select "Other Email Address..."
Enter the email address from which you wish to send the message and click OK.
Send the message when you have finished composing it.
1. If you do not have permission to send from the selected address, a message informing you of such will appear.

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