One of the big issues that any proposal of this type faces is how is
deployment going to take place alongside the old.
Fortunately this is actually quite practical if the SRV or NAPTR mechanisms
are used in the DNS. Basically the principles would be:
Users of mail-ng have a legacy SMTP server for accepting legacy
email
The existence of a new server is published by means of an SRV,
NAPTR or even possibly a plain TXT record with an SRV like
prefix.
One of the things that has consistently failed in the traditional legacy
infrastructure has been upgrading old protocols. Attempts to do this tend to
fall into the upgrade trap where everyone continues to send using the
obsolete version of the protocol in case they are speaking to a legacy
server,
result there is no incentive to upgrade.
Phill