Technical Detail
By altering an email's identifying fields, such as the From, Return-Path and Reply-To (which can be found in the message header), email can be made to appear to be from someone other than the actual sender.
Occasionally (especially if the spam requires a reply from the recipient, as in advance-fee frauds), the source of the spam email is indicated in the Reply-To field (or at least a way of identifying the spammer); if this is the case and the initial email is replied to, the delivery will be sent to the address specified in the Reply-To field, which could be the spammer's address. However, most spam emails (especially malicious ones with a trojan/virus payload, or those advertising a web site) forge this address too, and replying to it will annoy an innocent third party.
Prior to the advent of unsolicited commercial email (spam) as a viable business model, "legitimately spoofed" email was common. For example, a visiting user might use the local organization's SMTP server to send email from the user's foreign address. Since most servers were configured as open relays, this was a common practice. As spam email became an annoying problem, most of these "legitimate" uses fell victim to antispam techniques.
It is much more difficult to spoof or hide the IP or Internet Protocol address. The IP address is a 32 or 128 bit numerical label assigned to each device participating in a network and originates through the network provider making it more difficult to spoof or hide. Although this kind of verification is difficult for individual users, companies can use this technology as well as others such as cryptographic signatures (e.g., PGP "Pretty Good Privacy" or other encryption technologies) to exchange authenticated email messages. Authenticated email provides a mechanism for ensuring that messages are from whom they appear to be, as well as ensuring that the message has not been altered in transit. Similarly, sites may wish to consider enabling SSL/TLS in their mail transfer software. Using certificates in this manner increases the amount of authentication performed when sending mail.
Read more about this topic: Email Spoofing
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