A spam blocker is a special kind of software, software extension or even a simple technique that is designed to block unsolicited bulk emails, or spam. Spam messages are sent by unsolicited advertisers in bulk to email users.
Because many types of spam blockers are simply modified filters, a spam blocker also may be known as a spam filter.
Techopedia explains Spam Blocker
Some spam blockers are real programs, although small, while some are simply custom configurations in an email client’s filter function that check for certain keywords in the subject line of messages. This type of spam blocker has many limitations. Spam messages that get through the spam blocker are called false negatives, while legitimate emails that are blocked are called false positives.
Spam blockers and filters have lists that contain address that are either allowed to pass through or blocked. Addresses that are allowed to pass are placed on the "white list," while those that are supposed to be blocked are placed in the "black list."