I recently received an email with the "Disposition-Notification-To" header set. When I read the email, my Thunderbird client sent a "read receipt" to the sender to inform them I had read the email. I do not want Thunderbird to do this. How do I prevent it? Looking through the configuration options, the only vaguely relevant setting I can find is "Determine how Thunderbird handles return receipts". I don't know if this is the correct thing to set but, even if it is, it doesn't seem to work. Under "When I receive a request for a return receipt", it's already set to "Never send a return receipt". I also have Thunderbird set up to block remote content in the message to prevent html-based read receipts, so how is my client still sending them? I only know that this one was sent because the email address it sent to was faulty so I received a failed mail delivery message. Who knows how many others Thunderbird is sending that I'm simply not aware of!