Not knowing what avast version you are using (free/pro/AIS/Premier) hampers us a bit as the Free version doesn’t have an anti-spam function ?

If the entry is [Possible Spam] then that isn’t avast as the marking is *** VIRUS *** for a virus detected. I also have Thunderbird, but I don’t get any spam as I use MailWasher to filter my email before I download the rest. Unfortunately I haven’t seen any instance of this.

That is all I see in the avast free version Mail Shield Settings