Since we're listing issues with K9: It's pretty easy to setup if you know what you're doing, but the lack of any sort of autoconfig for non-Gmail mail servers make it very difficult to give it to a normal user and have it just work. Thunderbird has a standard autoconfig file to do this, and there is an open issue on K9 to add support for the same format, but it's been open and stagnant for years[0]

[0] https://github.com/k9mail/k-9/issues/865