Mailup is a German-based company that offers a swath of marketing tools, including email connectivity. Their target audience is large businesses who want a single tool to handle all of their business marketing: SMS, emails, CRMs, and everything in between.
Buttondown's focus is a little different! Buttondown is more interested in handling one thing — growing, managing, and sending newsletters — and doing it really well.
Buttondown is priced more affordably than Mailup, and is a better fit for small businesses and technology-forward companies who want to focus on their core product rather than spend all day futzing around in yet another piece of software.