Try to start *Windows* in safe mode with networking enabled.

- Win8 http://windows.microsoft.com/en-US/windows-8/windows-startup-settings-including-safe-mode

Still in Windows safe mode, start Thunderbird in safe mode.

- https://support.mozilla.org/en-US/kb/safe-mode

Does the problem go away?

Firstly thank you for taking up my cause. Booted into safe mode, and started Thunderbird in safe mode. Still same problem. However with one minor difference. The error message I get is usually repeated twice in the same box, however this time it was only repeated once. Not sure if that is significant.

What is your account type - POP or IMAP?

It is a Pop account

So that means messages sent should get copied to your local Sent file. What's the size of that file for your account?

I presume you mean the sent file in my pop folder under the thunderbird profile? About half a gig.

That's not huge but quite a bit.

First create a full backup of your profile. http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Backing_Up_and_Restoring

Then move messages from Sent to another folder, or e.g. create a yearly archive. Then compact the Sent folder and check the size of the file on disk.

Whether this will address your actual problem I don't know. But it's worth a try.

Moved most of the emails into a local folder. Left a few for my own operational reasons. Compacted the sent folder. Down to about 4MB now. Sadly though, still the same problem.

Considering your current line of questioning, thought I would go on the mail server just to check there was nothing blocking up that end. All clear. Nothing on the mail server.

I'm running out of ideas. Even though Windows safe mode should have excluded the possibility that AVG is involved, you may take a look at these steps regardless.

Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Don't let your anti-virus software scan incoming and outgoing messages.

Don't let your anti-virus software scan attachments.

Don't let your anti-virus software intercept your secure connection to the server.

Remove any add-ons your anti-virus software may have installed in Thunderbird.

Created the exception on the folder Switched off, in and outgoing scanning there were no add ons could not find how to protect the secure, so sent the email with my antivirus temporarily disabled.

Still the same thing.

Then I'm out of ideas. Maybe you can try to set up another account with a different provider (e.g. Gmail) and see if sending works better there.

Thanks for all the help. Do I have to repost this to get some fresh heads involved, or do I just wait to see if someone else can pick this up?

Up to you. If you want to start from scratch be prepared to go through the same drill.

If we leave it like this, will others pick it up, first time using the forums?

This is quite possibly an oldie but a goodie, and really should only relate to old modems or very low end stuff. Basically cheap or really old modems can sometime not manage to cope with the 128Kb chunks Thunderbird sends mail in. the following will reduce the chunks to only 65Kb. Five or six yeas ago this was a fairly common issue when Thunderbird first increased the buffer in Version 3 since then there is the occasional occurrence with new downloads and people that move house into premises with existing modems. But give it a go. No foul if it does not work.

On the toolbar > options > Advanced > General > config editor

• Click the "Config Editor..." button;
• Click the "I'll be careful, I promise!" button;
• In the window where the variables are displayed, right-click (or control-click, on a Mac) to get a contextual menu;
• Select "New->Integer" in the contextual menu;
• In the resultant window, type in "network.tcp.sendbuffer" and click "OK"
• In the next window, type in "65536" and click "OK"
• Close the Config Editor and Preferences/Options windows.

I might point out that despite discussion around the web on the mailtcp timeout, it really does nothing

Matt, thanks for your suggestion which made perfect sense and was a concern of mine, not because of an old modem, but I am sometimes concerned about the stability of my isp. I have modeified that right down to 32000, but still no joy.

Are you using a proxy? What anti virus?

Could you post the account settings part of the troubleshooting information on the help menu.

Could you log the SMTP connection. Perhaps that will generate clue For instructions see https://wiki.mozilla.org/MailNews:Logging#Windows