Maybe you are diagnosing problems with outgoing email. Maybe you are doing due diligence before choosing a web or mail host. Whatever the reason, here's how to check to see if your server is on a blacklist, and a few things you can do if it is. …
One way that email recipients minimize the spam they receive is through blacklists. A blacklist, a.k.a. DNSBL or RBL, is a maintained list of known spam sources. Blacklist users typically flag, score, or outright reject email originating from a listed IP. In other words, you don't want your mail server on a blacklist.
The Mx Toolbox blacklist check will check your mail server's IP against over one hundred blacklists, and provide links to details should you be on one or more. Just enter your domain name or mail server IP and press "Blacklist Check".
If you find you are on one, take action. Start with following the "Detail" link the blacklist tool gives you. As you do so, keep in mind that while blacklists do indeed contain mistakes or outdated information, they are usually right. Act with humility and start with the assumption that they blocked your IP for a good reason.
You may discover information that will require investigation with or action by your web host. Again, assume good faith. Approach them in a spirit of mutual assistance and skip the j'accuse drama.
If you share your IP address with other clients of your web host, you may find your address blacklisted due to another's actions. Reduce this risk and pay a little extra to have a dedicated IP address for your client, not one that is shared with your host's other customers. If you aren't sure if your IP is shared or dedicated, do a reverse IP lookup and see for yourself.
Again, start with the assumption that the problem is on your end. Be open to the possibility that, for example, your client has a misconfigured or compromised machine. Don't be a fool, however: pay careful attention to your host's response and don't hesitate to take your business elsewhere if warranted.
Check to see if you or your client are receiving bounce messages. Check the spam folder, if any, and not just the inbox. For example, one of my clients received this:
Subject: Mail delivery failed: returning message to sender
From: "Mail Delivery System" <Mailer-Daemon@sender.example.com>
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
SMTP error from remote mail server after RCPT TO:<email@example.com>:
host mail.global.frontbridge.com [184.108.40.206]:
550 5.7.1 Service unavailable; Client host [xx.xx.xx.xx] blocked using
Blocklist 1, mail from IP banned; To request removal from this list
please forward this message to firstname.lastname@example.org.
Take action as appropriate according to the bounce message's instructions.
Once you get the problem straightened out, consider signing up for Mx Toolbox's free mail server monitoring service, which will notify you of future problems.