Deleted

right click the folder, select properties and then the repair button. If that does not work try safe mode (hold shift while starting Thunderbird) and see if you have an add-on that might be the cause.

Thanks Matt, but as I said in my OP I have even resorted to a clean install. That included deleting everything including all folders (AppData/Thunderbird inc.) I did state that I'd tried repair etc. etc. etc. It still does it, even in 'safe mode'

In the third image which shows an email from this forum:

When you opened it and the top title bar says Order Update 936.. Does that 'subject: Order Update 936' relate to another email that you have deleted ?

No, it is from another email in my unified inbox, and from a different account. Deleting them seems to just move the problem to another previously OK email. If I re-sort the emails in the unified inbox (i.e. sort by size or whatever) the error shifts and swaps. If I go directly to the account listed under the unified inbox everything is sweet. It is almost like a pointer in a stack keeps pointing instead of moving (does that make sense?) It is only using unified folder where the fault seems to exist.

With the mention of unified mailboxes try the following.

On the toolbar > Help > Troubleshooting Information -> Show Folder button to display the profile in explorer or finder. Close Thunderbird

open the mail folder Delete the folder smart mailboxes in the mail folder. Restart Thunderbird.

That will force the recreation of the whole unified folder settings.

Did not make a difference, however there did not appear to be any data in the folders before deleting (see pic), even though there was mail in individual accounts that show up when using the unified view.

Change the view for your folder to All from unified. Have a play around among them. Is there a folder that also demonstrates this? If not

From the File menu (alt+F) select compact folders. Give it time to complete, say 10 minutes, just in case there is a lot going on. A couple should really do it.

Repeat the deletion of the smart folders (which should contains MSF files)

Still not working? I really have no idea what could be wrong.

Just in case the other does not work. It might shed some light.

Please add the troubleshooting information to your post To find the Troubleshooting information:

• Open Help (or click on three-line-icon and select Help)
• Choose Troubleshooting Information
• Use the button Copy to clipboard to select all. Do not check box "Include account names"!
• Paste this in your post.

Changed only domain names.

 Application Basics

   Name: Thunderbird
   Version: 38.5.0
   User Agent: Mozilla/5.0 (Windows NT 5.1; rv:38.0) Gecko/20100101 Thunderbird/38.5.0
   Profile Folder: Show Folder

             (Local drive)
   Application Build ID: 20151221142744
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 Mail and News Accounts
   account1:
     INCOMING: account1, , (pop3) pop.gmail.com:995, SSL, passwordCleartext
     OUTGOING: smtp.gmail.com:465, SSL, passwordCleartext, true

   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

   account4:
     INCOMING: account4, , (pop3) mail.mydomin.com:110, alwaysSTARTTLS, passwordCleartext
     OUTGOING: mail.mydomain.com:26, plain, passwordCleartext, true

   account5:
     INCOMING: account5, , (pop3) mail.mydomain.com:110, alwaysSTARTTLS, passwordCleartext
     OUTGOING: mail.mydomain.com:26, plain, passwordCleartext, true

   account6:
     INCOMING: account6, , (pop3) mail.bigpond.com:995, SSL, passwordCleartext
     OUTGOING: mail.bigpond.com:465, SSL, passwordCleartext, true

   account7:
     INCOMING: account7, , (pop3) pop-mail.outlook.com:995, SSL, passwordCleartext
     OUTGOING: smtp-mail.outlook.com:587, alwaysSTARTTLS, passwordCleartext, true

   account8:
     INCOMING: account8, , (pop3) mail.bigpond.com:995, SSL, passwordCleartext
     OUTGOING: mail.bigpond.com:465, SSL, passwordCleartext, true

   account9:
     INCOMING: account9, , (pop3) pop.gmail.com:995, SSL, passwordCleartext
     OUTGOING: smtp.gmail.com:465, SSL, passwordCleartext, true

 Crash Reports

 Extensions
   gContactSync, 2.0.11, true, gContactSync@pirules.net
   Lightning, 4.0.5.2, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}
   Provider for Google Calendar, 2.7, true, {a62ef8ec-5fdc-40c2-873c-223b8a6925cc}
   Send Later, 5.0.1, true, sendlater3@kamens.us
   ThunderBirthDay, 0.8.2, true, {4C9FE6FE-2C83-11DC-90B4-DC8456D89593}

 Important Modified Preferences

   Name: Value

     browser.cache.disk.capacity: 358400
     browser.cache.disk.smart_size_cached_value: 358400
     browser.cache.disk.smart_size.first_run: false
     browser.cache.disk.smart_size.use_old_max: false
     extensions.lastAppVersion: 38.5.0
     font.internaluseonly.changed: true
     gfx.blacklist.direct2d: 3
     gfx.blacklist.layers.direct3d9: 3
     gfx.blacklist.suggested-driver-version: 6.1400.1000.5218
     gfx.blacklist.webgl.angle: 3
     mail.openMessageBehavior.version: 1
     mailnews.database.global.datastore.id: 454ced4e-7c14-492f-a4df-29810c51e1d
     mailnews.database.global.views.conversation.columns: {"threadCol":{"visible":true,"ordinal":"3"},"flaggedCol":{"visible":true,"ordinal":"5"},"attachmentCol":{"visible":false…
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1465298243
     places.history.expiration.transient_current_max_pages: 40005
     plugin.importedState: true

 Graphics

     Adapter Description: Mobile Intel(R) 945GM Express Chipset Family
     Vendor ID: 0x8086
     Device ID: 0x27a2
     Adapter RAM: Unknown
     Adapter Drivers: ialmrnt5
     Driver Version: 6.14.10.4436
     Driver Date: 11-28-2005
     WebGL Renderer: Blocked for your graphics driver version. Try updating your graphics driver to version 6.1400.1000.5218 or newer.
     GPU Accelerated Windows: 0. Blocked for your graphics driver version. Try updating your graphics driver to version 6.1400.1000.5218 or newer.

     AzureCanvasBackend: skia
     AzureSkiaAccelerated: 0
     AzureFallbackCanvasBackend: cairo
     AzureContentBackend: cairo

 JavaScript

 Incremental GC: 1

 Accessibility

   Activated: 0
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

     NSPR
     4.10.10
     4.10.10

     NSS
     3.19.2.1 Basic ECC
     3.19.2.1 Basic ECC

     NSS Util
     3.19.2.1
     3.19.2.1

     NSS SSL
     3.19.2.1 Basic ECC
     3.19.2.1 Basic ECC

     NSS S/MIME
     3.19.2.1 Basic ECC
     3.19.2.1 Basic ECC

Would not having an account 3 make any difference??

I presume account3 was created and subsequently removed.

Could you check to see if account3 is still in your Profile folder name.

In Thunderbird Help > TRoubleshooting Information click on 'show Folder' button a new window opens showing contents of profile folder name. click on 'Mail' folder

check each mail account to see if it is account3. click on mail account name open 'Inbox' file - no extention using 'Notepad' emails are listed one after the other. Can you see an entry near the top that says 'X-Account-Key: account3' ?

If yes, then that mail account was the old removed account3. Close Inbox file.

Close Thunderbird now. Delete mail account name which has account 3 emails. Be certain you have the correct mail account or create a backup of folder on desktop before deleting.

then delete 'smart mailboxes' folder

Restart Thunderbird.

No account 3. Non msf files in smart mailboxes show 0 bytes?

There is something truly screwy going on. Smart mailboxes should not have a messagefilter file or an real mail (the files with no extensions)

Please look into your account settings within Thunderbird. If there an account in the list that specifies the smart folder as the location of it's "Local Directory in Server settings?

No, the accounts all show correct folders. I turn off unified folder view, close TB, delete smart mailboxes folder, restart TB and the smart mailboxes is not created. I then turn on unified folder view and it is created with all those files, except the msgFilterRules.dat. This is a totally new install from two days ago, including the profile.

msgFilterRules has reappeared it only contains the following

version="9" logging="no"

May be related...or not, but now it is creating files??

Have deleted the extra files and not been recreated...yet!

This latest version is haunted.

Title bar problem still exists as well.