Lets start at the beginning.

Thunderbird has a spam filter the learns from your choices. your filter does not help that, it actively hinders it. Try marking each message as spam before you delete it. If you account is IMAP then move it to the gmail spam folder and gmail will delete it. There is no point fighting the way gmail do their things.

Manual filters to remove spam are an endless cycle that I doubt you will ever get to the end of, other than making yourself a job every day modifying your filters.

There has to a limit to what can be added to a filter before things get unreliable and slow. Each mail has to be compared against each criteria in each filter as it comes in. That can only eventually get slow.

Matt, thank you for the reply. I'll follow your advice. However, there is still no explanation why the filter is not working. Is it gmail issue?

The filter runs before junk classification. Instead of deleting the message.... You could try: 'Set Junk Status to:' and ' Junk'

then in theory the Junk classification will run and put the email into gmail spam folder, assuming you have those settings set up.

Agree with Matt's comment.

Suggest you check the 'Account Settings' > 'Junk Settings' for the gmail account. With training, junk controls should be able to auto detect those emails you select as 'Junk' and auto move to gmails 'Spam' folder. good info here:

• https://support.mozilla.org/en-US/kb/thunderbird-and-junk-spam-messages