Agree it is not going to work if the spammer has messed about with the date.

Right click on 'spam' folder and select 'Empty Spam' is the manual method.

You could create a Message Filter you can manually run on Spam where 'Age in Days' 'is greater than' and choose a negative number like -3

If the emails are being downloaded to the 'Inbox' and Thunderbird is moving them to 'spam' or 'junk', then you could create Message Filter to run before junk classification using the same where 'Age in Days' 'is greater than' and choose a negative number like -3 then either perform action to delete or move them to a special folder you created which has a retention policy to delete all but the most recent X messages.

when i followed your proposal thats the result (watch attachement)

You need to find out whether it is Thunderbird or the server putting those emails into 'Spam'.

Do this to find the answer: Create a 'Message Filter' to run when 'Getting new Mail' and 'Filter before Junk classification' selecting 'Age in Days' and 'Is greater than' and select -3 and set the action to 'Move Message to' and select a folder you have created for this purpose.

When Message filter auto runs on the new incoming mail in Inbox, it moves emails with a date in the future to that designated folder.

If emails are being put in Spam by Thunderbird then the Message Filter would catch them and the emails would appear in that special folder.

If the server is auto moving them to 'Spam' then the Filter will not work because the emails were not in the Inbox. So emails will still appear in Spam'

The point is this. If you create that Message Filter to put those emails in another folder you created then you will find out whether Thunderbird is putting the emails into Spam or whether it is the server putting them into Spam.

If it turns out that Thunderbird is doing it, then the emails will appear in that folder. This means you could then modify the Message Filter to perform the action of 'Deleting the message'. This will then put it into the 'Deleted/Trash' folder which you can set up to auto empty on exit.

This does not help if the server is putting emails into Spam. But lets see what's going on first.

The image you posted shows you using the same setting in the 'Retention Settings' of the folder as I suggested for the Message Filter. The arrow selector to choose a number will not go in to the negative, but apparently is possible to manually insert a negative number which results in '4294967' being displayed. Clearly this is a bug as it does not accept negative, I bet this has something to do with how that number is stored - integer or string ?

I suggest you try: 'delete all but the most recent 1 messages.' If emails do end up in that folder, it would be useful to know if that retention policy works. But I'm still not hopeful if that is still linked to date/age in days as it would be the same problem as in 'Junk settings'.

In addition, I have made a request for an enhancement to the 'Account Settings' >'Junk settings' to provide a straight forward option to auto Empty/delete every X days. So that it is not based on date or age of email. You can follow if you like or it would be helpful if you could add additional comment or at least Vote for the bug. https://bugzilla.mozilla.org/show_bug.cgi?id=1673176