How can I create a filter? I want all mail containing a specific word, e.g “Fedora” to not be included in the unified mail, and instead in a specific directory. I receive many such mails and the unified inbox gets cluttered. Can I use Google’s label function? Or something else?
This not a functionality of K-9. If you want to filter (or to be more precise: apply rules), you must configure them on your server.
for google, you can do this with https://mail.google.com/mail/u/0/h/?v=prf in the wonderful old but fast and light html interface that is announced to be discontinued, in the desktop interface, it is here: https://mail.google.com/mail/u/0/?ui=2#settings/filters