Allowing a single user or domain to relay through Postfix

I'm running Postfix on a RHEL7 server. I've started to use a new iPhone to send email, and I'm seeing this in mail.log:

Oct 30 20:15:56 kyushu2 postfix/smtpd[31145]: warning: hostname ue.tmodns.net does not resolve to address 172.58.200.63
Oct 30 20:15:56 kyushu2 postfix/smtpd[31145]: connect from unknown[172.58.200.63]
Oct 30 20:15:56 kyushu2 postfix/smtpd[31145]: NOQUEUE: reject: RCPT from unknown[172.58.200.63]: 454 4.7.1 <[email protected]>: Relay access denied; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<smtpclient.apple>

My assumption is that Postfix sees me as trying to use timboyer.org as an open relay. I don't particularly want to allow all iPhone users to use my mail server as a relay. Is there a way to allow just @timboyer.org to relay?

Thanks,

Tim

NiKiZe, your answer is absolutely correct. I spent Sunday reading and was going to come back on with a 'never mind!' - I'm working on implementing SASL right now.

http://www.postfix.org/SASL_README.html#testing_saslauthd

Thanks very much for the pointer, and yeah, some spammer would have figured out the open relay thing eventually. :)