You mention 'Spam' folder (not 'Junk') so I'm presuming this is an imap mail account. Run through the following checks seeing if it resolves issue after each check.

First log on to your webmail account via a browser and see if there are any emails in the server 'Spam' folder. If it is empty, check to see if there are any settings that auto empty that 'spam' folder. If it is empty then that may explain why you cannot see anything in Thunderbird.

Then in Thunderbird: Check 'spam' folder has not been unsubscribed.

• Right click on mail account name in Folder Pane and select 'Subscribe'
• click on 'Refresh'
• select the 'Spam' folder
• click on 'Subscribe'
• click on 'OK'

Check the account is actually using that subscribed 'Spam' folder.

• Right click on mail account name in Folder Pane and select 'Settings'
• select 'Junk Settings'

Destination and Retention

• Select 'move new junk messages to'
• select 'Other' and select the 'Spam on mail account name'
• Do not select : 'Automatically delete junk mail older than x days'

Check the 'Spam' folder does not have any filters being applied.

• select 'Spam' folder
• Look at the 'Quick Filter Bar' to see if any of those icons are highlighted. If any are highlighted, click on them to remove that filter.

Check to see if the indexing file has gone awol:

• Right click on 'Spam' folder and select 'Properties'
• Click on 'Repair Folder' button
• click on OK

In same area - see if the following fixes issue:

• Right click on 'Spam' folder and select 'Properties'
• select checkbox 'When getting new messages for this account always check this folder'

Also check the 'Retention Policy' tab

• Select 'Use my account settings'

OR

• uncheck 'Use my account settings'
• Select checkbox 'Don't delete any messages'
• click on 'OK'

Please note: there are many servers who have adopted the policy of Emptying TRash and Spam folders where emails are older than x days. eg: gmail will empty trash every 30 days and also remove emails older than 30 days which are in the 'Spam folder. But not all server use same time frame. Other examples. Spectrum empty trash every 3 days and empty Spam every 14 days: https://www.spectrum.net/support/internet/how-delete-emails-bhn-twc/ Yahoo empty trash every 7 days and empty Spam every 14 days.: https://help.yahoo.com/kb/trash-spam-folders-regularly-emptied-sln3518.html

Server use Spam filters and often block spam email before they get to your Inbox.

Thank you for your reply, I appreciate it. Yes, it is an imap account, I'm italian so as spam I'm referring to "Posta Indesiderata". I checked everything you wrote and set like you said, but for the moment no messages back or new. Could it be because outlook changed its terms? Maybe in that process something wrong happen. When I login with the browser and go to the folder, it said that junk mail will be deleted after 10 days. Even if it is true, that day I have spam mail arrived within 10 days, so there is still no sense to the mail to vanish like that.