When an email is found to have malicious contents, or classified as spam, it can be added to a BLACKLIST. A blacklist is a list of emails, or domains, that are blocked from any server that uses that blacklist - essentially, a list of known spammers. When this happens, it often blocks any emails from that server, so if something@business.com is on the list, any emails ending in @business.com would be blocked. This can sometimes happen when a computer or other device has a mass email spamming malware that automatically sends spam. In this case, the first step is fixing the infected device.
There are many blacklists that are used by many email servers, so in order to get an email taken off a blacklist, the first step is finding which blacklist has the email. Some tools such as http://mxtoolbox.com/blacklists.aspx can search common blacklists for a specific domain. Once found, there are often ways to remove a domain from a blacklist, such as submitting a form. Usually, after a certain amount of time, domains are automatically removed.
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