The correct rule is that clients must not send message data if the reply begins with 4 or 5. This is enough of a benefit for me to recommend that all servers support EHLO, but there is no interoperability excuse for requiring it. It is unclear what this means. Some obvious interpretations prohibit the behavior of existing hosts. It is horrendously unclear what this means.
Some obvious interpretations prohibit the behavior of existing hosts.
This is garbage. This is garbage. There is no reason to demand that these clients use MX lookups. This problem seems to have been fixed in smtpupd-11. This is garbage.