Thunderbird has been great for me. With internal email systems, connection issues were usually due to proprietary protocols doing things in weird and undocumented ways that was the problem.

I've had a few issues with it for sure but still on it as nothing compares. Definitely not for the price. You didn't say what's incomprehensible about the UI, it seems fine to me. I don't need my email client to look slick(subjective anyway), I just need it to work and work well.