You have complete control over what Thunderbird does with junk/SPAM emails in account settings. Deleting them, except after a period of time is not one of the options offered there. That is because Thunderbird does not delete spam. It moves it to the junk folder and that is about it.

If the from address is in you address book, the default is to not ever consider that sender as SPAM, regardless of the message content.

So you are left with your settings at outlook.com as that is where MSN is these days. They have extensive settings to refuse email and block email from certain domains and senders, so I think you probably need to investigate things there. Try this link to your MSN Junk settings https://outlook.live.com/mail/0/options/mail/junkEmail

Thanks Matt. I had a look at the settings in Outlook, but I couldn't find Gumtree in the list of banned addresses. The strange thing is that I used to receive those emails, and Gumtree told me that they started being identified as spam in April. Why that happened I've no idea. I didn't identify them as spam. Regards, Denis.