Add some redundancy
There is currently only 1 MX record for mail.tutanota.de - please add 1-2 more for redundancy, there is always a possibility for system failure and sometimes it can take a while to repair it, so it might be better to have some more mail exchangers. Also when someone is misusing a server for spamming and it lands on some spam lists, you can just shut it off for a while and clean up and other mx'ers will take over. Also currently your IP addresses are provider allocated it's better to run a dual homing system with more than one providers and provider independent addresses, just for the case a provider is failing or you get DDOSed on one line etc.
Gute Idee! Wäre doof, wenn dadurch E-Mails verloren gehen.