The FreeBSD.org mail system implements some of the stricter Postfix checks on incoming mail and rejects mail that is either misconfigured or is potential spam. Your mail might bounce for one of the following reasons:
The email is being sent from a known spam domain or IP block.
The FreeBSD mail servers reject email from known spam sources. If you have service through a company or domain who generates or relays spam, please switch to a service provider who does not.
The body of the email only contains HTML.
Mail should be sent in plain text only. Please configure your mail user agent to send plain text.
The mailer at FreeBSD.org cannot resolve the IP address of the connecting host back to a symbolic name.
Working reverse DNS is a standard requirement for accepting mail from a host. Set up reverse DNS for your mail server's IP address. Many home services (DSL, cable, dialup, etc.) will not give you this option. In this case, relay your email through your service provider's mail server.
The hostname given in the EHLO/HELO part of the SMTP exchange cannot be resolved to an IP address.
A fully qualified, resolvable host name is necessary in this part of the SMTP dialogue before mail will be accepted. If you do not have a host name that is registered in the DNS, then you should use your service provider's mail server to relay your mail.
Your message had a message ID ending with the string ``localhost''.
Some mail user agents generate bad message IDs which will not be accepted. You will need to persuade your mail user agent to generate a valid message ID or else configure your mail transfer agent to rewrite them.