I suggest you try a real email service. One not "powered" by Microsoft exchange. Exchange has always been a corporate mail server with setting for the admin to over ride those of the user. I think you will find you do better with someone like GMX or Google. . Try it and see. I would be needed to file a bug anyway.

I don't have an email server... just using hotmail addresses direct off the web (now known as outlook.com). Is this a bug in TB? it seems a pretty basic thing to have been missed... always sending a read receipt despite option set not to do so. And very important for privacy/security. But what can I do to prevent these read receipts being sent out?

Try another email service. If you use email, you are indirectly making use of a server. That provided by microsoft (aka live, hotmail, outlook) is deficient in many ways.

If another email service does not exhibit this behaviour, we have evidence it is caused by a particular server and not an inherent defect in your mail client.

None of my many accounts in Thunderbird, from a variety of providers (including two via Microsoft) behaves as you're describing. Therefore it seems to me that this is specific to the server your email passes through.

It may also be useful to visit the webmail site to see if it has any settings for receipts.

It's not an option to change all my hotmail email addresses to something else.

So I just need my email client to be compatible with hotmail. I expected that of TB from its web site blurb.

Where is this rogue return receipt coming from? it seems TB, since if I open the emails exclusively via the web service all works as it should with no read receipts (with the web options being set to 'never send').

When TB opens an email it must be sending out a signal that the mail has been opened, and that triggers the return message. It should not be sending out that signal since the option 'never send' is checked, so how can it be prevented? If the options settings don't work is there something in the 'config editor' to tweak ? Or something else?

BTW I see now a second anomaly... when sending a message with TB, strangely TWO copies of it appear in the Sent folder; when sending via the web service only the one shows in the Sent folder.

Can you shed any light on all this?

Unfortunately you like many folk appear to view using the outlook web interface with using Thunderbird or another mail client as the same thing. They are not. The web experience is a web page, Thunderbird uses mail protocols. Outlook.com will also not respect the use of BCC when used by a mail client. That is certainly not the fault of the mail client but deliberate decisions made by Microsoft.

That Microsoft choose to use exchange server as their mail software makes then unique in the free mail provider arena. It also makes their mail service have all the oddities of their shoddy implementations of SMTP and IMAP / POP. Where the rogue receipt is coming from is Outlook.com in my opinion. Have you tried another mail service to see if you have the same issue with them. No you are arguing that you have a problem and some one should tell you how to fix it.

If you can replicate your experience with another mail provider then you have determined the issue is with Thunderbird. Until them my contention that it is being done by outlook.com is as good as any.

so how then, in your contention, would "outlook.com" know to send out this rogue receipt, if TB hadn't told it that it had opened the email? (the receipt only happens after opening the email by TB).

Actually it's your mistake to assume I'm like many folk...

I have already done plenty of testing of the various possibilities. I see nothing which points to "outlook.com" as a probable source of the problem.

No one is asking you to change your email address, but just add another one from a non-hotmail service. I'd suggest googlemail or gmx.

If that new account exhibits the same behaviour then yes we'd concede that Thunderbird is doing it.

But none of my accounts in Thunderbird suffer the problem you describe.