So we don't waste time, please tell us what you have done to try to solve this issue.

I first noticed the problem with lineone on my smartphone when I started to receive error messages 'queued' and 'not sent'. I searched the internet for the correct lineone server (smtp) settings and came up with 'tiscali' as well as lineone. (now owned by talktalk). I changed my mobile smtp setting from O2 to tiscali and all was back to normal. I did not realise for months that my 'sent' thunderbird emails for lineone (on PC) were not being delivered. No error messages or other indication of a problem. Extensive testing with many email addresses confirmed this. All my thunderbird accounts use a common outgoing server setting of smtp.blueyonder.co.uk and this had worked well with no indication of non-delivery from lineone. As a trial, I then changed my lineone settings to lineone and then tiscal then talktalk. All of these resulted in an error message from thunderbird (could not connect etc) I have checked other server settings but all appear correct. I also noted that the webmail address for lineone included tiscali.

Summary: there is no lineone problem with mobile and webmail but thunderbird will not send emails.

Perhaps I should describe what happens when I attempt to send an email to ANY address. Progress box comes up > progress bar completes. New box says 'message sent securely' Nothing is delivered.

I'm using version 52.1.0 but the problem predates this. Using Windows 10 fully updated.

I suspect you are free to use any smtp server with your lineone account. You need to find one that doesn't object to relaying.

I don't use my lineone.net account often, but it gave an error. Setting it up again gave a different group of settings that also didn't work, and a repeat of the same error saying that ESMTP smtp.talktalk.net isn't available.

Why did you stop using blueyonder? That is supplied by virgin and should be independent of lineone.

I haven't stopped using blueyonder. My other three thunderbird accounts all work well with blueyonder as outgoing server. I only changed the lineone settings because 'sent' emails were not being delivered from this account.

Changing the outgoing server solved the smartphone problem with lineone. Unfortunately this approach did not help my thunderbird account problem. I've run out of outgoing servers to try. I would like to re-install thunderbird but i'm worried about data loss.

The only way to prove conclusively whether or not Thunderbird is causing the problem you are having is to install another e-mail client on the same computer, and see if it works while Thunderbird does not.

If you want a clean sweep of using Thunderbird, then:

1. Backup the Profile folder. Thunderbird stores all personal data such as messages, address books, calendars and configuration settings in a hard drive folder called the Profile. This folder is separate from the installation of Thunderbird. Read about the Profile here.

2. Uninstall Thunderbird.

3. Delete the folder "Thunderbird" in C:\Users\**your username**\AppData\Roaming\

4. Run a Disk Cleanup

5. Reboot the computer.

6. Download & install the latest version of Thunderbird

7. For now, just set up that one e-mail account.

8. Send a test e-mail to one of your other e-mail accounts, and check that account for the e-mail, using your phone, or other device.

If there is still a problem, you will still need to prove that Thunderbird is the problem, by trying another e-mail client on the same computer. The SMTP server you are trying to use may not be working properly, or your computer IP address is blacklisted, and that is why your e-mails are not getting delivered.

If you want to restore your original Thunderbird Profile from your backup (with all it's settings and e-mail), then read this.

If you want to keep the new Profile and import the data from the old profile, then read this.

As you suggested I set up an account for my 'lineone.net' in another e-mail client. Windows 10 'mail app' seemed to be the most convenient.

I have to report that this new account sends and receives lineone emails with no problems. I have tested this with many email accounts and all seem to work well.

I can only conclude that there is a software error in my installation of thunderbird. I have used thunderbird for many years and it has been upgraded many times. Sometimes errors creep in. Thank you for the instructions regarding re-installation. This would appear to be the next step.

Just create a new profile and set up that e-mail address in it. If that works, then you know that the profile is the problem.

I removed and reinstated the lineone account in thunderbird using POP3. With the settings allocated (tiscali) I could not connect to the server, as before. However when I made blueyonder the outgoing server, I was able to send messages and they were delivered. So to a certain extent the problem is resolved. Smartphone, WIn Mail and webmail all operate with tiscali as the outgoing server while thunderbird will not. The thunderbird account is now functional and so I have no real desire to get further involved. Thank you for all your help