I've been loving Superhuman for years, and Kondo is exactly that for LinkedIn. Before I found Kondo, I didn't message people on LinkedIn very much, but now I find myself using LinkedIn a lot more simply because I have a good UX and can stay on top of replies.
I also want to shoutout Mitchell, the founder. I had a few questions when I signed up, and hopped on a call with me within 2 hours of my signing up. He watched me use it and gave me tips to make the most of the platform for my particular usage pattern. I've continued to send him my feedback on the product (via Kondo of course!)