Spam and RBL Systems
Many companies who maintain their own mail servers have had problems with RBLs and spam control software. A primary symptom of being on an RBL is the constant rejection of emails sent outside of the company, returned with 550 or 554 errors.
RBL stands for Realtime Blackhole List. Several companies and private entities throughout the world maintain these lists of who they think are spammers. What these lists contain are usually the DNS name of the mailer in question, their IP address, and the reason why they are on the list. The companies who keep these lists then in turn license the lists for use in spam control software. So, if you go out and buy anti-spam software for your computer, chances are an integral part of that software is an RBL database which is updated frequently by the company who keeps the list. If someone tries to send you an email from a server which is listed in the database, your anti-spam software rejects it and bounces it back to the sender with a 550 or 554 error. Usually the bounce-back reply will contain a message telling the sender that they are in an RBL.
Why are you in an RBL database? Well, the reasons can be any or all of the following:
- You have a mail server connected to the Internet using a dynamic DSL connection. Many spammers just set up mail servers on a simple DSL line and start sending out tons of spam. Now, anyone that sends mail on a common DSL line can arbitrarily be put in an RBL.
- Your DNS records are set up incorrectly. Spammers also tend to set up mail servers without DNS records, so that they are harder to trace back to the real sender. If your DNS records are not in order, or set up incorrectly, you can be placed in an RBL.
- You are running an open SMTP relay. What this means is your server is accepting connections so that anyone can use your server remotely to send out their spam without needing their own server.
- An email was sent to someone from your mail server, and several recipients (especially if they are AOL users) flagged the messages as junk mail. This is often reported back and your server becomes listed.
A common problem with RBLs is that many RBL systems talk to each other. Therefore, if you get listed on one RBL - chances are your information has been relayed to several RBLs. This makes it extremely hard to get your server removed from every RBL once it has been listed. Sometimes the process of getting your server removed from all of the RBLs can take weeks once your mail server problem has been corrected.
So, as a general rule of practice, companies setting up mail servers should make sure that they do the following to make sure that they stay off of RBL databases:
- Put the mail server on a line with a static IP address.
- Make sure the ISP who provides the IP address provides a Pointer Record and Reverse DNS.
- Register a domain name for the domain of the mail server, a proper MX record, and a proper A record for the server.
- Make sure the relay is set up to relay only for those who need access to the server.
- Check your server often with virus scanners and other utilities to make sure your server isnt sending out unwanted emails.
- Keep your server patched and maintained always.