Assuming that TLS is used (which the IETF considers to be necessary by
default, right?), then I think practically the same effect could be reached
1. Permit immediate STARTTLS, without waiting for the banner or EHLO.
2. Let the client send (AUTH+)MAIL+RCPT+DATA in one batch inside TLS.
3. Require TLS 1.3 support for compliant clients/servers.
Given that, a client should only need to wait for one roundtrip during
STARTTLS and for one roundtrip during DATA.
ietf-smtp mailing list