There are several open source mail server options available.
The only real new protocol is JMAP which is pretty good and I’d much rather more clients and servers support JMAP than create a new protocol altogether.
I can recommend trying out Mox as an email server. The author built everything from scratch and does a lot of great work when it comes to unifying everything. The wizard spits out all the discovery, DKIM, DMARC, DANE and MTA STS related subdomains you have to change the entries of (that is the most painful part) but everything else really works like a charm.
Much much easier installation process than weeks of trying to make postfix and dovecot work.
The only real new protocol is JMAP which is pretty good and I’d much rather more clients and servers support JMAP than create a new protocol altogether.