We plan to support DKIM for custom domains, but can not give a target date yet. It has been added to our Roadmap: https://github.com/tutao/tutanota/milestone/8.
Come on guys, please do that ASAP!
It would be great to have DKIM & DMARC for custom domains! Please make that happen :) It's a basic feature available through most providers, please make it happen for tutanota too!
This will be added with the new app by the End of April / beginning of Mai 2018.
Finallyyyyyy, waiting for this!