In my case I have to read a mail and it’s attachments to individually decide which rules to apply.
Most mailers rules or filters setup do only work or are triggered off the inbox automatically when they arrive as @Gary advised above as does eM Client. Same goes for online webmail rules & filters like Gmail.com & Outlook.com inboxes etc.
However you can allways manually run rules in eM Client if you prefer by right clicking. See the following thread for info on how to do that.
https://forum.emclient.com/t/rules-not-applying-manually/76446