Add DKIM and DMARC support for custom domains
DKIM prevents spammers and criminals from spoofing emails from my custom domain. Without DKIM it's easy for anybody to fake an email so that it looks like the message comes from me.
Spoofing lowers my domain reputation and it means genuine mail sent by me and other Tutanota users (through your servers) is marked as Spam/Fraudulent by Google and Microsoft.
Please implement this as soon as possible. You use it on the free Tutanota domain - think about your premium customers who use this for business purposes.