can I set an anti spam feature in K9-Mail ? I haven’t found a way to do it.
In Thunderbird, I also use Signal Spam Extension to report dubious mail messages. Is it possible with the eOS Mail App?
I don’t believe there is such a feature in K9-Mail unfortunately. I normally expect to do spam filtering ‘server side’ at my email provider’s site. I’ve had a quick look round in my /e/ account, and I can’t find a way to do it under the /e/ NextCloud mail implementation. You can add individual filters, but that’s not very helpful I’m afraid.
If /e/ are your only / main email provider, then maybe open an Issue asking them to implement it (though I’m not sure it’s even possible at the moment in NextCloud Mail).
If you have another email provider, contact them to ask how / whether they implement it.