The error is on the side of the receiving person's email provider. For some reason (and this could be any reason-could be blocking your domain, content, whatever), their side is blocking your emails.
You can get someone in your own domain -- same server, to send the guy a sample email also and see if it bounces back. If it does, that probably your outgoing mail server is probably blocked. OCheck with your email provider / sysadmin and ask to make sure your company email hasn't been blacklisted. If it doesn't bounce back, either you sent content that was filtered as something unacceptable, or your email address as been blacklisted by the receiving party or the receiving domain.
Also try sending an email from a neutral 3rd party email provider, such as gmail or yahoo.