What is it?
A blocklist (sometimes still referred to as a “blacklist”) is a list of Internet addresses and domain names that have been reported as sending out unsolicited advertising (aka “spam”), distributing malware, hosting botnets, hosting phishing websites, etc. Many e-mail services will not deliver mail from blocklisted servers. There are many public blocklists and each e-mail service may use any combination of them (or none at all) to decide whether or not to deliver your e-mail.
Why is it a risk?
If your IP address or Domain is listed on a blocklist, then e-mail you send is likely to be rejected by at least some receiving e-mail servers.
How can you mitigate the risk?
Many blocklists will automatically remove first-time offenders after a period of time (which varies). Repeat offences get progressively harder to remove.
The most important thing to do is to figure out why you were blocklisted in the first place and work to correct that problem. If you don’t, then your work to get de-listed will be short-lived.
The first step is to figure out which blocklist(s) your domain or IP Address is on. There are a great many blocklist checkers out there; most are free. In your browser search for “blocklist checker”
Each blocklist that your IP address or Domain appears on must be resolved individually, but most have an automated process that you can follow. Because (by definition) your email address is not trusted, they often have complicated processes for identifying yourself/registering.
Some blocklists are quite small and affect only a few users; you may decide it’s just not worth the effort. Others are quite widely used (e.g. Google, Spamhaus) and could impact your operations significantly.
If you have appeared on the blocklist previously, it may require proof that you have resolved the issue before successful removal from the list. You have no recourse in that “nightmare scenario;” you’ll need a new domain name or IP address. Here’s hoping it does not come to that.