This is a complex area, and rarely is Thunderbird the issue.

1. I assume we are talking IMAP mail accounts? Please confirm.
2. Are these folk sharing email addresses? We see a lot of that in this forum and it is always a bad idea.
3. Are the emails also being synced to a phone? turn that off to test if it is the source of the mysterious problems.
4. Is the Thunderbird profile folder being scanned by the anti virus? Create an exception to prevent that. It is not usually a good result either from a data loss perspective or not having Thunderbird stop while the AV scans a text file for threats. and when the UI catches up the user has deleted 4 mails or opened half a dozen write windows.
5. Does right clicking the folder and selecting properties and repair refresh the content without all the other actions being required?
6. What anti virus products are in use? Is there any commonality between users with issues if more than one?

1. Yes IMAP account 2. no accounts are not shared, one person always uses 3. Yes the same account is set to mobile phone, but this is exactly what IMAP accounts do in contrast to POP3. It is impossible to test this method because an account on your phone is needed. 4. e-mails are not saved to the local disk 5. it does not help. As I wrote above, only re-setting the account and synchronization solves the problem. 6. the anti-virus program is ESET. Yes, there are several users who have the same problem, but I do not think the cause is on the side of the anti-virus program. I am currently testing MS Outlook and there is no problem there, so the reason lies in handling IMAP in Thunderbird.

Ok. You have a problem no one else has you are not interested in attempting to diagnose the problem. Instead suggesting "It is impossible to test this method". That is not even remotely true. It might be inconvenient, but it is not impossible.

What exactly are you expecting from a support forum with fellow users attempting to assist you that can not replicate your mysterious issue? I certainly can not replicate the issue with the information you have supplied.

You could log the connection and when you identify the event that makes things get out of sync you might want to come back with that information so someone can replicate the issue.

https://wiki.mozilla.org/MailNews:Logging

There are also two users where they do not use e-mail on the phone and they have the same problem.

OK after few tests. This is example on my account. Only on TB one message from March 16th is not displayed (currently the only one from that day). imap log

In att screenshots from TB and another mail client.

Please test: Does going offline by forcing a synch get missing email to display? File > Offline > Download/synch now

If you have set up Thunderbird to auto synch all folders when going offline then you could click on the 'two blue screen icon' in bottom left Status Bar. This will take you offline. clicking on the 'Two black screen icon' immediately takes you back online.

Please report back on results.

19/5000 Nothing has changed.

Is the imap account set up to download only headers or full copies of emails for the folder(s) which contain missing emails?

It is set up to download full copies of emails.

Today another invisible message joined for TB. Anothers emails clients - no problem.

Which folder view are you using?

'Menu icon ' > 'Folders' > 'All' or 'Unified' ?

If 'Unified' please swap over to 'All' and use that view for a few days to test.

Always was "All"

... and ? I'm still waiting for resolving or suggestions? ... any ?

My two suggestions are 1. Disable ESET - see what happens 2. Try beta https://www.thunderbird.net/en-US/channel/ - backup you profile first

Missing email is your only problem?

Yes missing emails is my big problem. ad. 1 - nothing - I've done it before ad. 2 I can test the beta on my unit. Employees' computers - a bit risky

new version - it did not help

Can you look at missing email 'Subject' to find out what is a common factor with missing emails, for example:

• Is the email always a 'Reply' and not a new email subject?
• Does the Subject always start with something like PD:[undeliverable]?

Please confirm you do not have 'threading' enabled in the 'Sort by' options.

- both - the above example has PD [undeliverable], but the only filtering is after the date.

any another suggestions?

Start/boot up your computer in 'Safe Mode with Networking' mode and use only that mode for a few days to test.

This is also a test your fellow co-workers could also perform.