Just to confirm I've understood correctly, you are saying: if new mail arrives on the Server whilst Thunderbird is closed eg: overnight, then when you start thunderbird and it does the initial synchronising all seems ok, messages are still unread upon the first synchronising with server.

But if you have Thunderbird running and you are reading older mail and new mail is detected on server, all subsequent new mail appears as marked as read.

If you read an email via webmail account or phone or another computer then it is marked as read on server and so anything then downloaded to the Thunderbird mail account will obviously be marked as read.

Do you access server via webmail, phone or other computer etc? Does anyone else have access to same mail account maybe via a different computer? Do you use any message filters which can mark as read? Is there any setting on the server/webmail account which selects anything downloaded to be marked as read?

This might sound crazy, but.... Have you ever used 'Outlook' as an email client on your computer to receive emails for this same email address/mail account? If yes, suggest you open Outlook and remove the mail account, Outlook may be interfering and marking the emails as read.

Thank you for answer.

Your problem (re)statements are correct.

I understand your 3rd paragraph: If you read an email via webmail account or phone or another computer then it is marked as read on server and so anything then downloaded to the Thunderbird mail account will obviously be marked as read. and that is how it should work - being marked as read if the email was truly read on any of the other devices -- but that is not the problem.

I did check that no other other devices interfere. We have two smartphone also accessing the account. I turned them off but still get emails automatically marked as read.

Also checked filters -- and as far as I can tell I have no active filters (image of filter dialog is attached)

About the 'crazy' Outlook idea: I did have Outlook connected to the account earlier on but had deleted the account in Outlook and Outlook is no longer running. To double check, I activated the email account in Outlook again and then deleted it again. This did not change the behavior of marking incoming emails as read. As far as I can tell, Outlook is disconnected from the email account.

I'm convinced this is not Thunderbird as it is only showing what has been determined on the server. It is likely to be either the smartphone or backup to icloud or another app working in the background.

RE: webmail account. Access webmail and check to see what email is marked unread.

RE phone Are you accessing using pop? Because ISP server may mark downloaded as read on server regardless of whether you have read them or not. You do not necessarilly need to read an email to get marked as read on the server.

Located some info that people using an auto backup to icloud was causing emails to be marked as read. Also, Outlook app and Yahoo Mail app can seem to work in the background and cause issues. It is best to remove them. https://discussions.apple.com/thread/5362651?tstart=0

https://www.slipstick.com/problems/smartphones-mark-mail-read/

There was a recent case where the user was not getting notifications for new mail, which I think would also happen in your case when TB is running and new messages appear as read. In the former case, an old Outlook profile was the cause, but in your case it might be necessary to adjust some setting in TB to account for the properties of the IMAP server.

Thanks for the suggestions ... I am positive that the effect is not caused by another device as I had everything turned off except the laptop running TB. Also I don't use any type of cloud backup - use old fashioned networked hard drive with manual backup :-)

Seems the most likely candidate now is the MS Office and Outlook installation. I will uninstall MS Office and see whether that helps. Will be a few days before I get to that ... work calls