Those screenshots are inconclusive. TB looks for the email id that is associated with server definition. Having a legitimate email address that is not defined will not be accepted. Your mention of 'edit' isn't clear. What happens if you click to compose a new message and insert the nondefined email id? My guess is the message will not be sent from that id. Please give more information.

Hi David,

Thanks for your reply.

The left hand side of the second png shows the Sent folder and that my email was sent from my valid but not defined in Thunderbird email address. That email address is indeed valid, I have received many emails into and sent many emails from this address.

Here's an incoming email that proves that email address is valid. What other info do you need?

Thunderbird won't allow send, but that email address is valid-3.png

warm regards, Cris Saturday, 2022/02/12

Still inconclusive. That image you mentioned shows two screens, one of which shows an error message indicating message was actually sent from a different id. Again, click to write a new message and use the undefined email id and report what happens. TB (and other email clients) depend primarily on ISP for defining a proper incoming email and on the SMTP settings for outgoing emails.

Hi David,

Thanks again for your reply.

The image #1 showing two screens has red circles showing that it's a draft message from cbxu3a@LSn and when I try to send it, Thunderbird refuses saying it will only send it from cbx@LSn.

I did write a new message as you suggest and used the undefined-in-Thunderbird email address cbxu3a@LSn. Image #2 left side showed the sent folder with red circles showing the email address it was sent from proving that it was indeed sent from cbxu3a@LSn and it had already been sent.

Sorry, I see the right part of image #2 which I didn't close from the previous snapshot is confusing the issue. That right part of image #2, apart from explanatory highlights the same as #1, shows my attempt to send but Thunderbird refused with the error message at the bottom - it does NOT show that the email was sent, only that if I were to send it, it would only send it from my cbx@LSn address. Sorry for the confusion, I should have closed that.

warm regards, Cris Saturday, 2022/02/12

That seems to further show the issue. Maybe start over. Write a new message from undefined message and send it and report what happens. You're wanting TB to send message from an email id that is not defined. That would only work if ISP was not particular on who is using its SMTP servers. My guess is yours is set for authentication, requiring email id and password. If That is allowed for one email, it will be allowed for all.

Yes it does indeed show the issue, so why start over?

The real problem in Thunderbird is that when I reply to an email sent to me at a valid but undefined-in-Thunderbird, it does not put that email address but only one it knows as valid. But it's impossible to receive an email to an email address unless it is indeed a real one, whether or not Tbird knows about it or not.

Here in #4 I'm replying to an incoming email to me at cbxu3a@LSn. But Tbird is a pain, it changes my email to cbx@LSn rather than using the one in the email I'm replying to. see:

Tbird sending problems-4.png

And it sends no problem from the cbx address.

Now in #5, the image with the yellow unique identity rejection message, I'll repeat that, but this time I'll revert my address to cbxu3a@LSn and then save, close it, reopen the draft and there's now no send, I have to edit the email to get the send button.

Tbird sending problems-5.png

Hope this helps explain the problem, or rather the two problems.

warm regards, Cris Saturday, 2022/02/12

Here's the 5th image, I got the dreaded waiting ball for the 5th and eventually gave up, sent it without and now here it is...

It still won't attach it, don't know why, it's only a screenshot at 402 kb.

Okay, let's let it go. TB is not supposed to send email that is not defined in SMTP settings, but if it works for you, that's fine. If there is a bug, it would be that it works at all, not that it should work consistently. And I remain an unbeliever, but that's just because I'm not seeing it directly, as you are.

Just to be clear: SMTP settings are what defines the sending email address. Not the incoming address. that is true, not just for TB, but for others. TB is doing you a favor by allowing a reply at all.

Thanks David, but as I said before:

"The real problem in Thunderbird is that when I reply to an email sent to me at a valid but undefined-in-Thunderbird, it does not put that email address but only one it knows as valid. But it's impossible to receive an email to an email address unless it is indeed a real one, whether or not Tbird knows about it or not."

So this is actually a bug, and needs to be fixed... How do I report this problem as a bug in Thunderbird?

warm regards, Cris Saturday, 2022/02/12

Cris, Your ISP is providing the email that TB requested. TB has no control there. For example, I have several aliases defined with my ISP and if I ask TB to retrieve messages, the ISP hands over those with my email id and also any that used an alias. This is not TB in action; it's your ISP. TB is involved only on the sending. I hope this helps.

Hi David,

Thanks for your reply.

Since my ISP is providing the email address that TB requested, why does it not use that email address when I reply to an incoming email with a valid but not-declared-to-TB address?

TB is an email receive and reply system, it receives emails from my ISP and needs to be able to reply to them. Rather than "TB is doing you a favor by allowing a reply", replying to incoming emails is a major TB function and as such should operate seamlessly. But it doesn't.

A reply in TB should always use the address on the incoming email, but it doesn't. Is there any reason for a check on whether an email address is correct other than to ensure that an address actually works? Since it's the address on an incoming email, it does obviously work and so how can any further check be either needed or desirable?

So this issue makes TB less than user-friendly and so needs to be reported as a bug. This desirable change would, of course, make TB become more user-friendly.

How do I report this bug?

warm regards, Cris Monday, 2022/02/14

Cris, The decision to provide a message from ISP to TB is done by the ISP, not TB. On the outgoing side, the ISP has a different situation: it wants to ensure that only authorized customers use the SMTP server. And the authorized customer is the email address defined in the SMTP server settings. My guess is that other email address of yours is possibly defined within your ISP as an alias, and alias ids are valid for incoming, but not for outgoing. To say that it is a 'bug' that TB does not reply to incoming email id would be a gross misstatement of what you are experiencing. If you want to receive from multiple email ids, either define them as aliases, or set up additional accounts and use Unified View to simulate a virtual inbox.