- start Thunderbird in safe mode

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

- go to tools | addons - remove the lightning addon - restart - start thunderbird - if you need the lightning addon, add it back at tools | addons

Does that help?

I tried safe mode with Lightning disabled and again with Lightning and Theme and Font (the only two tweaks that I use ) disabled. In both cases, the test messages disappeared after closing Thunderbird and re-starting with the message still in the Inbox.

I've looked in the /Thunderbird folder but can't see where the missing messages are going.

Referring to my earlier comments, why is there an INBOX-3.sdb folder? I have only two imap accounts (imap-mail.outlook.com and map-mail.outlook-1.com) I also find that since restarting in safe mode, the first of those two accounts is also missing the corresponding INBOX folder but retains the mysterious INBOX-3.sbd folder. I am unable to attach a screenshot to show the anomaly.

DexterB said

I tried safe mode with Lightning disabled and again with Lightning and Theme and Font (the only two tweaks that I use ) disabled. In both cases, the test messages disappeared after closing Thunderbird and re-starting with the message still in the Inbox. I've looked in the /Thunderbird folder but can't see where the missing messages are going. Referring to my earlier comments, why is there an INBOX-3.sdb folder? I have only two imap accounts (imap-mail.outlook.com and map-mail.outlook-1.com) I also find that since restarting in safe mode, the first of those two accounts is also missing the corresponding INBOX folder but retains the mysterious INBOX-3.sbd folder. I am unable to attach a screenshot to show the anomaly.

If you created subfolders in the Inbox then it would create an Inbox.sbd folder to hold those created subfolder files. I would not worry about it. What is a bit odd is that it has a number. Usually they are called just 'Inbox.sbd' because they are the subfolder container for the 'Inbox' file. If there was an 'Inbox-3' file, then I might understand it.

However, in the lower (imap-mail.outlook.com account, I see an Inbox.msf file which is the indexing file, but there is no Inbox (mbox) file.

It would seem that you have not chosen to synchronise that subscribed folder or it has been removed.

Anti-virus program may have quarrantined the file. do you have an AV product auto scanning your TB files?

In Thunderbird right click on mail account in Folder pane and select 'Settings' select 'Synchronization & Storage' for the imap mail account Using info at the following link, please check details - eg: do you want to synchronise the Inbox ? Is it selected to be synchronised ?

• https://support.mozilla.org/en-US/kb/imap-synchronization#w_configuring-synchronization-and-disk-space-usage

I did previously note that there was not a corresponding INBOX folder for one of the accounts. I do not have any sub-folders, nor have I set any up previously. Why inbox-3.sbd is there is a mystery to me. I'm guessing that's less of a problem to me than the absence of an INBOX folder for one of the accounts.

Synchronisation is the same for both accounts (see attached screenshot below).

I do have A-V, but it's not reporting any quarantined files. I will try disabling it just in case.

I'm slowly forming the view that the only way out is to delete the dodgy account entirely and re-installing it. I'm still losing mail.

Thoughts, advice or suggestions?

As this is an imap mail account, all emails are stored on server. You could remove the account via Account Settings. select the mail account you want to remove. click on 'Account Actions' and select 'Remove account'. Click on OK

Then set it up again to reconnect to those folders. Via 'Menu Bar' 'File' > 'New' > 'Existing mail account' or Via 'Mail Toolbar' 'Menu icon' > 'New messages' > 'Existing mail account'.

Suggest you make a note of the server settings you are using - just in case you need them.

Follow-up: I "bit the bullet" and deleted the rogue INBOX-sdb folder I've been whining about. It didn't seem to change anything. Thunderbird still kept losing inbox messages as before. I was reluctant to nuke the Thunderbird account and re-create it as suggested above. I feared losing previous messages (Yes - I had saved the profile and know how to re-instate it, but, i have developed a profound suspicion about these strange goings-on and don't trust Thunderbird any more).

I set up a new account on Mac mail (an exact duplicate of the Thunderbird account). I had never used Mac mail before, but it worked OK as expected. Strangely, even though I didn't migrate any folders from Thunderbird to Mac mail, some (but not all) turned up in Mac mail anyhow!!

To deepen the mystery, I then discovered that the Thunderbird profile got it's missing INBOX folder back. Now, Thunderbird works for my account (unlike before) but won't send messages on my wife's account (that hasn't changed since "the troubles" began).

It seems to my empirical eye that Thunderbird is a fair bit quicker in send and receive than Mac mail, but Mac mail appears to work for both accounts. My wife is accustomed to Thunderbird and wants it (but working fully for her). I'm happy with either client, if it works.

I'll play around a bit more to see if I can make Thunderbird work for her but if not it's over to Mac mail. A pity really - I've been a staunch Thunderbird follower for many years and reluctant to switch, but it just isn't cooperating any more.