No reverse DNS configured
If you see the following error message appear in your log file:
Jun 22 11:56:42 es01 postfix/smtp[11369]: 1A53BD80A4B: to=<[email protected]>,
orig_to=<[email protected]>, relay=mail.example.com[1.2.3.4]:25, delay=0.65,
delays=0.21/0.01/0.27/0.16, dsn=4.0.0, status=deferred (host
mail.example.com[1.2.3.4] said: 451-9.8.7.6 is not yet authorized to deliver
mail from 451 <[email protected]> to <[email protected]>. Please try later. (in reply
to RCPT TO command))
this can indicate that you did not yet configure a reverse DNS entry for the IP-address of your mailserver. Activate the reverse DNS (test it with host IP-address). Probably the original message was greylisted by the receiving mailserver, so Postfix will retry to deliver the message in a couple of minutes.