"Because of missing DNS records for http://Facebook.com, every device with FB app is now DDoSing recursive DNS resolvers. And it may cause overloading ..."
I was a paying Tutanota customer for two full years, but ended up cancelling recently. Reasons being lack of IMAP (ProtonMail does), slow development, [missing features](https://old.reddit.com/r/tutanota/search?q=missing&restrict_...) yet their focus is on building quantum cryptography defense, and most importantly, their recent change in domain limit. It used to be unlimited on the 12 EUR plan, but without any announcement to the end-user, they limited that to 1. They [fixed that later](https://old.reddit.com/r/tutanota/comments/lwr2ag/important_...) but I no longer trust their business practices.
You can have an independent login account but that comes at an additional cost of 12 EUR. That's on the top of the Business plan's cost of 12 EUR + 12 EUR.
Speaking for calendar, I basically signed up for a new Google account with that custom domain email address and use calendar on it. Syncs neat to my Android.
"Because of missing DNS records for http://Facebook.com, every device with FB app is now DDoSing recursive DNS resolvers. And it may cause overloading ..."