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K9 does not appear to be under continued development. The last version on Google Play is from 2018. By contrast, I received several updates to FairMail just in the past week.
As for "free" or not, my feeling is that we need to support developers. If someone has spent a lot of time developing an app that I use all the time, the least I can do is send him/her several $ to support continued development. So I have no problem paying/donating for apps I use.
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