As I do not get any help. My solution was to put mail into archive. Maybe others who have this problem can do same. It is not ideal but it is a bit better.

Sorry it seems a while.

I've been checking out same issue on imap gmail account and also seems to keep downloading all mail and not the selected 30 days. I had deleted all necessary files from the profile folders with thunderbird not running. I've put this forward as a potential bug and now waiting to see what the response may be.

See this bug report. Comment 6 makes an attempt at documenting how this feature ("Synchronize the most recent X days") really works: https://bugzilla.mozilla.org/show_bug.cgi?id=1564421#c6

The above bug report contains much testing and info on the process. The following info should help you to understand what is going on.

re :'synchronise the most recent 30 days' What does this actually mean? It does not mean you will only see 30 days worth of emails in the Message List. This will still download all headers of all emails in any subscribed folder, so you would still see the full list of emails, but it does not necessarilly mean the full copies of emails have been downloaded. Synchronising is about what is being downloaded and stored on your PC in mbox files and not necessarilly about what you see in the 'Message List'.

If you have not selected any folders to synchronise: In 'Account Settings' > 'Synchronisation & Storage' uncheck checkbox : 'Keep messages in all folders for this account on this computer' Click on 'Advanced' button and make sure no checkboxes are selected.

Then there will be no mbox file, so all emails shown in list will only be headers. This means there will only be index (.msf) files in the profile folders, so keeping the amount of space used on computer low. You cannot use this in offline state nor as a backup of emails.

Repairing the folder will ensure it is up to date and accurate. Selecting an email will force contact with the server to download a temp file allowing viewing of email. Going into offline mode will basically mean you cannot read any emails because there is no synchronising of any folder as non have been selected. Instead you would probably see something like this in the Message Pane: The body of this message has not been downloaded from the server for reading offline. To read this message, you must reconnect to the network, choose Offline from the File menu and then untick Work Offline. In the future, you can select which messages or folders to read offline. To do this, choose Offline from the file menu and then select Download/Sync Now. You can adjust the Disc Space preference to prevent the downloading of large messages.

The following situation occurs when you have selected one or more folders to synchronise with server. In 'Account Settings' > 'Synchronisation & Storage' uncheck checkbox : 'Keep messages in all folders for this account on this computer' Click on 'Advanced' button and select checkbox for eg: Inbox click on OK select 'synchronise the most recent 30 days' click on OK

Exit and restart Thunderbird.

You will still see all emails in the list regardless of date, but the last 30 days worth of emails in the Inbox folder (that folder being selected to synchronise emails) have now been fully downloaded and will be stored in an Inbox mbox file. So athough it does not look any different in the Message List, it is completely different in the profile folders.

An Inbox mbox file has now been created and it will only contain full copies of the last 30 days of emails.

If you select emails that may be older than 30 days, they are downloaded and stored in the mbox file and marked as 'pending deletion', thus increasing the mbox file size. Compacting the folder will remove those emails marked as 'pending deletion' from the mbox file honouring the setting 'synchronise the most recent 30 days', thus saving space on your PC, but they are not removed from the server and they will still be visible in the Message List because the headers are still downloaded.

File > Offline > download/sync now This will download all copies of all emails into the folders selected to synchronise with server eg: Inbox. This will increase the mbox file size as all emails have now been downloaded. The effects being that you can use it to backup emails or go into offline mode and still be able to see all emails.

However, in the process of doing this, all those additional emails older than 30 will have a 'pending deletion' marker. In online mode right click on Inbox folder and select 'compact' and those emails will be removed from the mbox file honouring the setting 'synchronise the most recent 30 days', thus saving space on your PC, but they are not removed from the server and they will still be visible in the Message List because the headers are still downloaded.

So if you do not want to see older emails in eg: Inbox then you should move them to an appropriate folder.

You can choose to unsubscribe from seeing folders you created. OR you can select specific folders as 'Favourite Folder' Right click on folder and select 'Favourite folder' and then use the 'Favourite' view to restrict what folders you see in the Folder Pane. Menu icon > Folders > Favourite

Menu icon > Folders > All This gets you back to the standard view.

Hopefully, this info offers a better understanding of what is occuring when you select to synchronise folders.