Spam Email, also known as junk email , is a subset of electronic spam involving nearly identical messages sent to numerous recipients by email. The messages may contain disguised links that appear to be for familiar websites but in fact lead to phishing web sites or sites that are hosting malware. Spam email may also include malware as scripts or other executable file attachments. Spam is any unwanted and harmful mail. Separation of spam from normal mails is essential. This paper surveys different spam email filtering techniques. The different techniques are Machine learning based, list based, content based and hybrid or other. Machine learning based, is mostly used because of high accuracy and mathematical support.