Those are hidden preferences and there is no consolidated list of what they are, or what they do. So I suggest you start Googling each of the preferences to find a description or search the source code. https://searchfox.org/comm-central/source/mailnews Or you could save a lot of time and simply use the tick box in settings (search receipt and click the to request receipts button) <img src=https://assets-prod.sumo.prod.webservices.mozgcp.net/media/uploads/images/2024-06-23-17-26-47-eb3343.png> But I need to make this exceedingly clear. There is no requirement in the mail specifications for a return receipt to be sent when mail is received. I do not send them and really do not know of anyone that does. Some corporate environment which carter to the whims of executives on a power trip enforce sending receipts and it is an option on some server mail implementations to enforce them, but there is no such requirement imposed on internet mail clients and as you can see Thunderer has options for the user to set, not only about requesting them, but also on returning them. There is historically a delivery notification, which is different to a return read receipt, but again there is no requirement for them o be returned when requested and after the issue some providers have had with being blacklisted for sending them most mail servers now have the option disabled on the server. For a discussion on the backscatter problem that really saw the demise of delivery notifications see https://en.wikipedia.org/wiki/Backscatter_(email) So if you are looking for an old style signed delivery ala the paper postal service, none exists. Email delivery is simply not well suited to legal situations where proof of the recipients receipt of a notice is important.