Although most well-known for phishing purposes, there are actually several reasons for spoofing sender addresses:

• Hiding your true identity, although if this is the only goal it can be achieved easier by registering anonymous mail addresses.
• Easy to rotate. If you are spamming, you are bound to be blacklisted quickly. If you’re able to switch sender addresses, who cares?
• Pretending to be someone the receiver knows. This can be used to ask for sensitive information or just plain orders to transfer funds.
• Pretending to be from an organization the receiver has a relationship with. Phishing attempts to get hold of bank login details etc. are the most common example.
• To give the sender a bad name. Sending out insults or other messages that put the so-called sender in a bad light.
• Identity theft. Being able to send messages in someone’s name can be the start of an identity theft procedure.

One way to spoof emails is if the evil-doer finds a mail server that has an open SMTP (Simple Mail Transfer Protocol) port.

SMTP itself lacks authentication so servers that are poorly configured in this way are prey to abusers. And there is nothing that can stop a determined attacker from setting up his own email server.

Having done that there is – freely available – software that will allow you to use any sender address you like. The receiver would have to check the full headers of the mail to find out whether the mail came from the “real sender” or if it was spoofed. This takes some knowledge and time, that you probably do not want to spend on every incoming mail. In these cases however replies go to the actual handler of the email address and not the attacker.

That is why, in cases like CEO/CFO fraud you will often see that the attackers registered a domain very similar to the one of the company they were trying to trick.

A difference in the domain that could be easily missed by the intended victim, like for example Ka$mare$.com.That will enable them to get any replies from their victim in case they were asked for more information or confirmation.