Headers are not scanned for killfile until clicked on.

when a filter created that is based on subject or sender, then any new articles are automatically filtered with that filter. However, if an filter is created that is based on some other characteristic in the headers, then it is not automatic until the article is clicked on.

Example:

%BOS
%Score created by Pan on Wed Mar 11 21:59:49 2021
[*]
Score:: =-10
User-Agent: G2/1.0
%EOS

I want to filter out messages from Google Groups since they are the leading producer of spam. However until I click on the article first, the article doesn't get the -10 score.

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