Your message did not reach some or all of the intended recipients. Subject: RE: New Classes Sent: 2/18/02 5:40 PM The following recipient(s) could not be reached: 'Recipient Name' on 2/18/02 5:40 PM There was a SMTP communication problem with the recipient's email server. Please contact your system administrator. <originating.host.addr #5.5.0 smtp;591 your host is blacklisted by blacklisting.host.addr. Send your questions to firstname.lastname@example.org>it means you've gotten caught by the net we've spread to fight spam. If you're here reading this, it means that most likely you are trying to get some legitimate email to one of our users. We apologize for the inconvenience.
To the user trying to send the email: We have refused the email to our user because it was coming from a network host which has been blacklisted. There are two reasons the host could be blacklisted: either because it is listed as an "open relay" (that is, a host which spammers can use as a tool to deliver their unwanted messages to many users), or because it has been identified as a source of spam itself. The lists we use are not maintained by us. It is not under our power to have the host you were using removed. Your best course of action would be to contact your system administrator who can determine where and why the host is blacklisted and get things straightened out. The most common method of contacting someone who can take care of this would be to send mail to your postmaster. If your email address was email@example.com, then your postmaster's email address would be firstname.lastname@example.org. Forward your bounce message to him. If it's not apparent to him what action he should take, you could suggest he visit this web page.
In the meantime, if it's urgent that you get an email to the person you're trying to contact, you can send the message to the person given in the "Send your questions to" line of the bounce message you received. That person will be able to forward email on an emergency basis.
To the sysadmin trying to remedy this situation: In order to fight spam, our mail server uses two blacklist services, mail-abuse.com, and zen.spamhaus.org (as well as some home-made ones). Depending on which of those has blacklisted your host, you should visit their websites (go to their FAQ page) to learn how to get off their list.
And thanks for taking care of your machine!