DSN (Delivery Status Notifications) - according to IETF.org RFC5337
Please implement the IETF standard according to RFC5337 (and others) for DSNs (Delivery Status Notifications).
It enables us to have a proof of delivered emails in any case, which is very useful in technical diffucult and legal and other cases.
Posteo.de and other services have implemented it already very successful. Please do so as well.