I’m receiving errors and bounces when sending email, what do they mean?

You have users who complain that emails are not being delivered. Emails are rejected with errors from the Kerio Connect server and make mention of "RBL" or "blacklist" errors.

Errors are typically of the form:


These errors appear to be sent from Kerio Connect, but they are not. They are errors other mail servers returned to Kerio Connect while trying to relay your outgoing mail. Kerio Connect is simply passing the errors back to the sender. The remote mail server reporting the error should be mentioned somewhere in the error message usually as a internet name or an IP addressAn identifier assigned to devices connected to a TCP/IP network. or both. The most important information from these errors is the name of the RBL you are reportedly listed on.

Your mail server IP addresses is likely listed on other RBL (Realtime Black List) lists also. A RBL is a DNSDomain Name System - Enables the translation of hostnames to IP addresses and provides other domain related information. service that provides a reverse lookup on IPs that have been reported as SPAM sources or open relays. It is likely that you are mistakenly listed, or your mail server is configured with an open relay and is visible on the internet.

Determine which RBLs your IP is listed on

Find out which RBLs your IP address is listed on so you can take steps to remove yourself. Use an RBL checker such as the one at rbls.org. There are plenty of RBL checkers on the internet. They are an essential tool for this kind of troubleshooting.

Take steps to get removed from the RBLs

If an IP address is listed on an RBL, that RBL must be dealt with on an individual basis. They are individually maintained by various organizations and companies. They each have a website you can visit or contact information with instructions on how to request list removal.

Check the IP for open relay

If the mail server has is open relay, it's internet IP address will constantly be listed on RBL lists. They will usually remove the IP at your request, but it won't take long to be listed again unless the open relay is fixed.

To identify if you are an open relay, refer to KB Detecting that Kerio Connect has been compromised and used for spamming.

Check for private blacklist

Finally, if you are not listed on any known RBL list, it is possible you are listed in the remote mail server's private blacklist. Some ISPInternet Service Provider - an organization that can provide Internet service.'s have aggressive private blacklists that might accidentally include small domains who are innocent. Fix this by contacting the postmaster at the remote domain and ask to be removed. Mail servers typically accept email sent to postmaster@domainname. Sometimes the bounce message contains specific information on how to be removed so read the message carefully. It might be necessary to send any email to the remote domain from another email address outside of the blacklisted domain.