As an email marketer, you should avoid being on a blacklist at all costs. Being on the blacklist means your domains, email address, and IPs have been blocked due to lack of trust, leading to a hard bounce. A leading cause of blacklisting is a messy email list. Once blacklisted, your subscribers will never receive your emails in their inboxes because Internet Service Providers (ISPs) and Email Service Providers (ESPs) have blocked you.
Everybody now takes security seriously on the internet, including email providers. To keep their users safe, email providers tend to block (blacklist) any domain perceived to be sending spams or malicious content as emails to their users. It is, however, important to note that email blacklist is just one of the many issues that may prevent your followers from getting your emails.
An email blacklist is a real-time list of IP addresses or domains believed to be sending spam. Organizations such as internet service providers (ISPs), free mailbox providers, and anti-spam vendors use these to avoid spam from reaching their networks.
Websites are not left out of the blacklist phenomenon, with some web browsers programmed to block potentially dangerous domains automatically. You can read the full article here.