The core email protocols do not have any mechanism for authentication, making it common for spam and phishing emails to use such spoofing to mislead or even prank the recipient about the origin of the message.
When an SMTP email is sent, the initial connection provides two pieces of address information:
Together these are sometimes referred to as the "envelope" addressing, by analogy with a traditional paper envelope, and unless the receiving mail server signals that it has problems with either of these items, the sending system sends the "DATA" command, and typically sends several header items, including:
The result is that the email recipient sees the email as having come from the address in the From: header; they may sometimes be able to find the MAIL FROM address; and if they reply to the email it will go to either the address presented in the From: or Reply-to: header - but none of these addresses are typically reliable, so automated bounce messages may generate backscatter.
Although email spoofing is effective in forging the email address, the IP address of the computer sending the mail can generally be identified from the "Received:" lines in the email header. In malicious cases however, this is likely to be an innocent third party infected by malware that is sending the email without the owner's knowledge.
Email spoofing has been responsible for public incidents with serious business and financial consequences, such as an October 2013 email to a news agency, spoofed to look like it was from the Swedish company Fingerprint Cards, saying that Samsung offered to purchase the company. The news spread and the stock exchange rate surged by 50%.
Malware such as Klez and Sober and many more modern examples often search for email addresses within the computer they have infected, and use those addresses both as targets for email, but also to create credible forged From fields in the emails that they send, so that these emails are more likely to be opened. For example:
In this case, even if Bob's system detects the incoming mail as containing malware, he sees the source as being Charlie, even though it really came from Alice's computer; meanwhile Alice may remain unaware that her computer has been infected.
In the early Internet, "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, these sorts of "legitimate" uses fell out of favor.
When multiple software systems communicate with each other via email, spoofing may be required in order to facilitate such communication. In any scenario where an email address is set up to automatically forward incoming emails to a system which only accepts emails from the email forwarder, spoofing is required in order to facilitate this behavior. This is common between ticketing systems which communicate with other ticketing systems.
Traditionally, mail servers could accept a mail item, then later send a Non-Delivery Report or "bounce" message if it couldn't be delivered or had been quarantined for any reason. These would be sent to the "MAIL FROM:" aka "Return Path" address. With the massive rise in forged addresses, Best Practice is now to not generate NDRs for detected spam, viruses etc. but to reject the email during the SMTP transaction. When mail administrators fail to take this approach, their systems are guilty of sending "backscatter" emails to innocent parties - in itself a form of spam - or being used to perform "Joe job" attacks.
The SSL/TLS system used to encrypt server-to-server email traffic can also be used to enforce authentication, but in practice it is seldom used, and a range of other potential solutions have also failed to gain traction.
However a number of effective systems are now widely used, including:
To effectively stop forged email being delivered, the sending domains, their mail servers, and the receiving system all need to be configured correctly for these higher standards of authentication. Although their use is increasing, estimates vary widely as to what percentage of emails have no form of domain authentication: from 8.6% to "almost half". For this reason, receiving mail systems typically have a range of settings to configure how they treat poorly-configured domains or email.