A blacklist is a list of IP addresses or domains that are suspected of sending spam. Email servers use blacklists to help decide if they should accept or reject an email. When used correctly, blacklists help keep the bad stuff out of inboxes.
Blacklists identify spammers by the IP address or domain they send email from.
An IP address is a string of numbers that identifies the network of any device connected to the internet. It's like a return address on an envelope, and is associated with a human-readable domain. Since the IP address tells other devices where data is coming from, it can be a good way to track bad content.
Blacklists can list a single IP address or a range of IPs. Internet service providers (ISPs) reference these lists to block potentially malicious emails from their customers. MailChimp monitors and handles any major blacklisting.
What is a greylist?
A greylist identifies the IPs or domains of bulk senders. This list is useful if a private domain does not want to receive email marketing of any kind. It's not unusual for MailChimp to show up on these lists because we are a bulk email provider. Although many greylists won't remove you once you've been added, being included on one doesn't usually affect email delivery.
What about greylisting?
It can be confusing, but the term "greylisting" refers to an unrelated anti-spam technique. Greylisting happens when a server blocks delivery and asks the sender to send the email again in a few minutes. This is a popular way to fight spam because a legitimate sender will wait to try delivery again but spammers often don't.
How blacklists affect delivery
Anyone can set up a blacklist for their own email server and share it with the public, so not all blacklists are created equal. Smaller blacklists don't have much influence with bigger internet service providers, and unless your server references that blacklist, you won't see any bounces.
Certain blacklists carry more weight and are more likely to cause bounces. We actively monitor reputable blacklists, including (but not limited to) Spamhaus, Invaluement, Barracuda, Frontbridge, SURBL and the URIBL blacklist. If a MailChimp server ends up on one of these lists our Delivery Team investigates the cause and get us removed as quickly as possible.
Larger ISPs often have their own internal blacklists that cannot be looked at by outsiders but can cause bounces. We pay close attention to our delivery rates for those domains so we can prevent blocks and quickly resolve any problems that come up.
Blacklists are useful, but they're just one part of email delivery. Keep your lists up to date and engage your contacts to help make sure your email is delivered.
I think I'm on a blacklist
Blacklists exist to do good spam-fighting work, but they're not perfect. Sometimes blacklists confuse legitimate senders for spammers. If you find yourself on a blacklist, don't worry, it's usually temporary. There's not much you can do, but most of the time it won't affect your email delivery.
So, what happened? IP addresses can end up on blacklists for lots of different reasons. A sudden increase in email volume can be a red flag for blacklists. Or, if a MailChimp user sends an email campaign to an stale list of people who forgot they ever opted in, a large percentage of those people could report the email as spam.
Those types of complaints could cause the MailChimp server to be temporarily blacklisted. Plus, if someone in an IP range has been identified as a spammer, the whole range might be added to a blacklist.
A lot of blacklists automatically remove addresses after some time has passed without any spam attempts. Some smaller blacklists might not have a removal option, but they aren't likely to be referenced. MailChimp keeps an eye on major blacklists and takes action for quick removal.