To diagnose problems with Thunderbird, try one of the following:

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

If this is an IMAP account, are drafts saved on the IMAP server on in Local Folders?

Andrew and sfhowes thank you for your replys. I will work through your diagnostics in due course Andrew and by way of information sfhowes all of my accounts are POP Mail Server type.

It appears that despite the error messages the email does in fact get saved as a draft message but when I attempt to send the message I get the following:-

"There was a problem including the file file:///C:/DOCUME~1/Peter/LOCALS~1/Temp/nsemail.html in the message. Would you like to continue sending the message without this file?"

I checked that path and the file exists, it's an html copy of the message content with a broken link that is the jpg image of my signiture. When I choose to send the message without this file it does so and a perfectly complete message is received by the recipient and a copy moved to my sent folder.

I have TB on three computers and only have this problem on one so guess it's a system issue.

Please do try out the methods as provided. From what I conclude, a clean reinstall may be your best option, but let's try out the non-destructive options first.

That error message is usually the result of allowing an antivirus program, e.g. Avast, to scan the mail. Excluding the TB profile folder from AV scans is the general remedy.