How do I enable ESMTP support on my server?

Question

I've been running OSX Server for several months now and all works well.  The only frustration is that inbound ESMTP messages always seem to be rejected.  This can delay inbound mail by several hours at times.  A typical log message is
NOQUEUE: reject: RCPT from dabinett.apple.com[17.151.62.52]: 450 4.7.1 <xxxxxxxxx>: Recipient address rejected: Service is unavailable; from=<[email protected]> to=<xxxxxxxxx> proto=ESMTP helo=<bz.apple.com>

Answer 1

That's likely not something that will be resolved with ESMTP, that's probably greylisting. 
Greylisting is an anti-spam technique, as legitimate mail servers will retry and will resend mail messages when the target mail server is 450 busy, where the spam engines do not normally include this retry logic.  (Google for further details.)
You can choose to disable greylisting, or to whitelist specific domains, to adjust the thresholds for known domains, or to allow the server to continue to learn about the domains that are contacting your domain using its default setups.
FWIW, ESMTP is typically used for authorized clients that will be sending outbound mail via your mail server, and is not typically used for other mail servers that are communicating with your mail server.
To check the settings, launch Terminal.app and issue the command postconf -n and look for the smtpd_recipient_restrictions entry.  You will probably find something akin to this entry:
smtpd_recipient_restrictions = permit_sasl_authenticated permit_mynetworks  reject_unauth_destination check_policy_service unix:private/policy permit
To disable greylisting, enter these two commands:
postconf -e 'smtpd_recipient_restrictions = permit_sasl_authenticated permit_mynetworks  reject_unauth_destination permit'
sudo postfix reload