Saturday, February 4, 2012

Email server in blacklist

When email server is in CBL (composite blocking list), you can not send email to or etc because they will consider it is spam. Different blocking list (black list) provides the lookup utility and explanation of why your email server (SMTP) is listed and how to delist it.

Let's take CBL( as an example which has a lookup tool ( which will do IP lookup and explains the possible reasons you are listed in CBL, and how to fix that before you delist. When you are listed, there are several possible reasons:
  1. Spambot or botnet in email server which sends spam emails. Try different virus scan tools, or turn to tcpview for network activities
  2. SMTP configuration issue, like open proxy or open email relay is ON
  3. DNS configuration issue, like A record or MX record, do some online SMTP test (
  4. Diagnose using which checks against ~100 known blacklists (of course, including CBL)