Some times we see some messages to be rejected by postfix with erro RCPT from unknown
unknown[some ip address]: 550 Client host rejected: cannot find your hostname
if you do
dig -x your ip +short
you will get some hostname as the dig output. (reverse dns)
Reverse DNS of your ip exists and points to some hostname, unfortunately the same host name does not exist in forward dns file. That is the reason why Postfix regards your host as “unknown”.
Don’t use reject_unknown_client in your postfix configuration. It may reject the valid mails also.