I was at a customer site who was having issues sending emails from an IMAP client (Thunderbird in this case). Receiving emails was fine. The user could send & receive emails using Outlook & Outlook Web App (Yes, the client was running Exchange 2010).
Everytime an email was sent from the IMAP client, a pop up window appeared with the following error message.
“Cannot send the message. The mail server responded: 550 5.7.1 Client does not have permissions to send as this sender”.
Since the user was able to send using other clients, I was pretty sure that something was missing on the Exchange server. I knew that POP & IMAP clients uses the “Client (ServerName)” receive connector. The issue was that “ms-Exch-SMTP-Accept-Authoritative-Domain-Sender” permission was missing on the receive connector for authenticated users.
In order to allow POP & IMAP clients to submit messages via this connector, you need to run the command below.
Once the permission was set, users were able to send emails from the IMAP clients.