Do you have an antivirus program scanning outgoing mail? That is a frequent cause of timeout issues as they are just plain slow.

I spent a lot of time working with AVG, sending them logs etc. to determine if they were blocking it.

After reinstalling the account in Thunderbird, I believe the problem relates to some sort of 'cache' that Thunderbird maintains, a list of 'to do' items for each account, the user requested download all messages, the user sent a message, the user compacted folders, etc. and it doesn't fully process the send e-mail request if there are other items uncompleted ahead of that in the queue. I do some light programming but am not involved in the Thunderbird freeware development. It did send a message right after loading the account fresh. This also blocks sending messages on 'all' Hotmail accounts, not just the one with the backlogged queue.

Assumption is generally not a good idea.

I have no idea what the backlogged queue you are talking of is. What I do know is AVG has a reputation these days of being little better than the virus it is supposed to be saving your from. Totally disable email scanning is the first step. Regardless of what AVG are telling you.

Booting in safe mode with networking and attempting to replicate the issue is the second step.

The third step is log the SMTP connection. https://wiki.mozilla.org/MailNews:Logging

Then there is the recent issue with hotmail/ Microsoft where IP V6 is causing issues and folk need to disable that to get the Microsoft offering of a mail product to login correctly.

What I mean by the queue is does Thunderbird process requests sequentially or can it process more than one request at the same time? If it is in the middle of downloading a huge number of messages, IMAP, to the local computer, is it waiting until that process is complete before it 'responds' to the Outlook server which has already responded to the request to send the e-mail. I base this on the error messages which say something about waiting on client input. I am not assuming anything, just wondering how the program is working.

I did try, once, disabling AVG, before contacting them. They had me run a little program that generated a HUGE log that they wanted to see, but then it seemed all they did was check the ports, which I had already done.

I will look into logging Thunderbird from the link you provided. And I will try disabling AVG again.

Thunderbird's getting and sending of mail is asynchronous. The only time you see lockouts is if the IMAP connection and other devices have already used all available connections to the server (logins.) This occurs in recent times on Yahoo as they have apparently made some changes on total connections. Thunderbird will use up to five on an IMAP server. This can be set in Account settings> server settings > advanced to a lower number. If you have an Apple product in the mix, they will just keep trying to connect until the server refuses more connections. (A real good way to lock out competitors and frustrate folks really)

Then you see some other odd things with some providers, like, and this is just my observation, refusal to connect if it is less than 5 minutes since you connected last (for any reason) (That was hotmail BTW) That applies even if you click get mail twice in quick succession the second connection will fail.

I blame antivirus as a knee jerk reaction because they are so often guilty of messing things up. The more "integrated" the more they can quietly mess up. Things like Norton's "sonar" preventing the new account wizard probes to detect server settings, or scanning of secure connections without fully installing the necessary certificates to decrypt and encrypt so no mail arrives. These things have nothing to do with Thunderbird, but that is where folk see the failures.

Go to this link (I assume it works for all outlook account types) and check the server names and ports match what you have in Thunderbird.

You might also want to "try" changing the authentication method to oauth. (Normal password is generally what it is set to.)

If you are using two factor authentication (2FA), create an application password in the outlook website for Thunderbird to use. Trying to use 2FA with a mail client without is truly a disaster.

Ran the debug and tried sending e-mail. Image attached of results.

Well, I am finding the debug log very interesting and like learning more about what is going on. But this morning I decided to test again turning off AVG and trying to send. And it worked! So I will be contacting AVG again, on the same thread I was previously discussing this with them, and ask if there is another log that I can run directly in Windows so they can see what it is that AVG is blocking. Maybe it relates to that 127.0.0.1 local host issue. I did find a few others online asking about that over the years. Thanks for the help in any case. You may have been right suspecting AVG all along.

This is really nuts! After composing another e-mail to AVG, and including the link to this thread, I tried sending the message, AND IT SENT FROM THUNDERBIRD! Who knows what is going on, I will follow up with AVG when it is being blocked again. Thanks again.

Failing to send again this morning. Will follow up with AVG and press them on the issue. Wonder why they haven't gotten anyone else reporting the same problem?

For what it is worth, decided to set up accounts in Microsoft Office Outlook and the send function does work with AVG anti-virus enabled.

Still have not heard back from AVG.

There is something going on with the way they interact with Thunderbird sending e-mail over Hotmail.

I have used Thunderbird and AVG for many years, the problem only started in the past few months.