Interestingly, they use the Nodejs-based Haraka MTA instead of a more conservative choice like Postfix or Exim. I wonder what exact advantages they traded in more mature and certainly not much less performant software in for?
A very good and even cheaper alternative to this service I can recommend is mailbox.org, run by German IT service specialist Heinlein. They deliver a full OpenXchange setup (mail, calendar, contacts) with custom domain, 3 different mail aliases and additional + aliases and good privacy policy for 1€/month. They are very realiable (using a more conservative MTA, postfix), and the company is mature. They provide better value for money than migadu.com just by looking at it.