I downloaded the free version of MailWasher from Firetrust Ltd., a few years ago, which highlights spam messages, on my main email account.
When 'washed', messages are transferred to my Thunderbird mail agent in-box, with the spam messages being sent to the Junk/Spam folders.
You have to intervene when first used, to teach it your good incoming senders addresses, but it seems to recognise spam very quickly.
Although the free version is meant to cover only one email account, and there is a 'Pro' version for multiple accounts, I have 3 of my email addresses listed, which all seem to be washed by the free version.