What kind of email accounts do you use, POP or IMAP? Did you create the same accounts on another PC or a smartphone?

I'm not much of a geek, I think they are POP. I use Xfinity email that downloads to Thunderbird. I've checked my Xfinity account, I haven't made any changes-they disappear as quickly in Xfinity email page as Thunderbird. I really don't understand a lot of technicality with email

Which are the "Server Settings" for this account?

I have a Mac desktop forgot to mention that. I do not see Return receipts anywhere

Look carefully at my picture: It is not Return Receipts but Server Settings

I see nothing about return receipts in Server settings. I will continue this tomorrow

Why are you looking for return receipts? Who told you to search for return receipts? They have nothing to do with your problem.

I will resume this tomorrow-very tired,