I have approx 12 years worth of emails

Are you saying you do have 12 years worth of emails in your Inbox?

What's the size of your Inbox folder?

What is your anti-virus software?

Do you have a good backup of your Thunderbird profile folder?

christ1 said

I have approx 12 years worth of emails

Are you saying you do have 12 years worth of emails in your Inbox?

What's the size of your Inbox folder?

What is your anti-virus software?

Do you have a good backup of your Thunderbird profile folder?

Yes - I have 12 years of emails I use Kapersky AV My inbox is 34,000 messages - but only 420kb in size My back up is 1.68Gb in size I have a back up from 3 days ago of my profile (from mozbackup)

Most likely your anti-virus caused some sort of corruption of your Inbox. Before anything else follow these generic suggestions to avoid problems with anti-virus software.

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.

Then restore your good profile backup.

In case it isn't obvious, it isn't best practice to accumulate messages in Inbox. Keep your Inbox as tidy as possible. You may want to take a look at the Archiving feature. https://support.mozilla.org/kb/archived-messages

Also see: Keep it working. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

And last but not least, backup your Thunderbird profile on a regular basis. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

Note, MozBackup isn't supported anymore. http://mozbackup.jasnapaka.com/

"MozBackup is not being developed anymore. There are known issues and there is no time on my side to fix all issues and develop new features. Use MozBackup only on your risk."

christ1 said

Most likely your anti-virus caused some sort of corruption of your Inbox. Before anything else follow these generic suggestions to avoid problems with anti-virus software. 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. Then restore your good profile backup. In case it isn't obvious, it isn't best practice to accumulate messages in Inbox. Keep your Inbox as tidy as possible. You may want to take a look at the Archiving feature. https://support.mozilla.org/kb/archived-messages Also see: Keep it working. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird And last but not least, backup your Thunderbird profile on a regular basis. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile Note, MozBackup isn't supported anymore. http://mozbackup.jasnapaka.com/ "MozBackup is not being developed anymore. There are known issues and there is no time on my side to fix all issues and develop new features. Use MozBackup only on your risk."

I simply reinstalled (overwrote) TB - That sorted it out Thanks anyway

Don't be surprised when the problem comes back.