This is a technique used to block emails based on the content which appears in the from line, subject line, or the body copy of an email. Filtering software search for key words and other indicators that identify the email as potential spam. This type of blocking occurs on per email basis. Filters are also sometimes used while sending emails. You can “filter” depending on the content, business mails or personal mails and so on. You can also use filtering to sort your emails.