There is another threat and it is not noticeable by people because they are never aware of it. This threat is known as email threat i.e. the mail is being intercepted as it travels to the recipient.

  If you use Internet-standard e-mail servers, every message you send travels in plain text on an unpredictable path that can pass through dozens of intermediate computers or routers before it reaches its destination. At any step along the way, your message can be intercepted and read; it can also be altered. In fact, it's easy for a moderately tech-savvy crook to forge your name and address on a message so that it appears to have come from you. Because of that fundamental insecurity, you should never send confidential information such as credit card details or your Social Security number in a normal e-mail message; likewise, you should never rely on ordinary e-mail messages for important business transactions.

  Mails can be protected from prying eyes by using strong encryption and digital signatures so that the recipient can be certain the message was sent by you and hasn't been tampered with.

   And then there's the dark side of e-mail—unsolicited commercial e-mail, more popularly known as spam. For most of us, spam is a nuisance rather than a serious threat to our computer's security. But spam can carry viruses and other hostile software. Unwanted ads for Web-based casinos and get-rich-quick schemes can cause embarrassment or threaten your job security if they land in your work mailbox.