I think you’re on to something.
I also think there’s a difference in where the network effect kicks in for different types of social media. IMHO, Lemmy has just enough activity to not feel empty, and even then I wish there was more comments to interact with and more niche communities. With Pixelfed, I feel like as long as there’s enough interesting posts it makes sense for people to visit regularly.
IMHO, you should consider doing more troubleshooting on Kmail. I’ve never used it personally, but from my understanding, it’s a stable program and shouldn’t have problems doing the basics of email, like you’re reporting.