Proxmox Mail Gateway verify receivers

You can enable recipient verification in PMG by enable below option under Mail Proxy -> Options -> Verify Receivers.

If encounter below Domain not found error, make sure you have setup a proper MX record for your SMTP server in your local DNS server.

Jul 10 08:43:26 pmg postfix/smtpd[2588]: connect from superad2.com.sg[116.12.55.236]
Jul 10 08:43:26 pmg postfix/smtpd[2588]: NOQUEUE: reject: RCPT from superad2.com.sg[116.12.55.236]: 450 4.1.2 <xxx@mydomain.com>: Recipient address rejected: Domain not found; from=<smikl@superad2.com.sg> to=<xxx@mydomain.com> proto=ESMTP helo=<vps.superad2.com.sg>

http://www.postfix.org/postconf.5.html#reject_unknown_recipient_domain