There are hundreds of blocklists available, but not all blocklists are created equal. Because anyone can start a blocklist, a lot of them are not widely used by mailbox providers. This makes it difficult for a sender to know when to be concerned about an IP or domain showing up on a blocklist. 

The following blocklists are widely used because they are reliable and professionally maintained. Getting listed on one of these blocklists may lead to widespread deliverability problems, so be sure to find and fix the root cause of the listing. 

IP-based

• Spamhaus (SBL)
• Spamhaus (XBL)
• Composite Blocking List (CBL)
• SpamCop (SCBL)
• Passive Spam Block List (PSBL)
• Lashback (UBL)
• Invaluement (ivmSIP, ivmSIP/24) 

Domain-based

• Spamhaus (DBL)
• Invaluement (ivmURI)
• Spam URI Realtime Blocklists (SURBL)
• URIBL

There are some other influential blocklists around the world. Being listed on them may not cause widespread deliverability problems and may only affect a certain geographical region or certain mailbox providers.

To determine the severity of the blocklisting and if it is important to you:

• Monitor your deliverability and reputation in Everest to determine if your blocklisting is causing problems.
• Check your SMTP bounce logs for messages indicating filtering or a block due to a blocklisting.
• Know which mailbox providers you send the most volume to on a daily and monthly basis. If a blocklisting is only affecting a small portion of your list, then it can help you prioritize your investigation efforts.