The message was rejected because the other end's SMTP server did not identify itself with a valid hostname.
The sender's administrator can fix this easily. The sender's admin should create a hostname to match the host, or configure their email server to identify itself with a real hostname. Any customer or remote user can supply us with full headers of a bounced message by fax or email, and we can help pinpoint what should be fixed, and/or provide free and valuable advice for the other administrator on fixing this.
Our previous mailserver was lax about the rule, and it resulted in needless spam coming in because spammers don't follow any rules for operating email servers. Prior mailservers we've operated have been both lax and properly enforcing of this, so this is not a completely new change. Enforcing this rules greatly reduces the amount of incoming spam.