Thunderbird 78.10.0 (64-bit) My Exchange account configuration in Thunderbird: Return Receipts --> Use my global return receipt preferences for this account --> When I receive a request for a return receipt --> Never send a return receipt Thunderbird is ignoring this configuration as it always sends back a return receipt. For example: # Write up a test email to yourself # Options --> Return Receipt # Send # Wait for the email to be received in my Inbox # Open the email --> at this point the return receipt is send # After a few moment, the return receipt email is received in my Inbox I have even tried changing my account configuration to switch from "Use my global return receipt preferences for this account" and to set the account directly to "Never send a return receipt" but that makes no difference at all. I came to know about this because of a spam email that included the following headers: Disposition-Notification-To: <infos@commandes.site> X-Confirm-Reading-To: infos@commandes.site Return-Receipt-To: infos@commandes.site Exchange couldn't deliver the return receipt as the email address was no longer valid. Because of that I received back a failure notification from Exchange. Is there any particular reason why Thunderbird is ignoring my account configuration? This is quite annoying as even spam email can trigger this behavior despite me explicitly not wanting them to know I opened it in the first place (just like we're not loading external content).