No reverse DNS configured
If you see the following error message appear in your log file:
Jun 22 11:56:42 es01 postfix/smtp: 1A53BD80A4B: to=<[email protected]>, orig_to=<[email protected]>, relay=mail.example.com[126.96.36.199]:25, delay=0.65, delays=0.21/0.01/0.27/0.16, dsn=4.0.0, status=deferred (host mail.example.com[188.8.131.52] said: 451-184.108.40.206 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.