Correct, you should not pay the invoice!

CEO fraud

Explanation: It is suspicious that the e-mail is not sent from an official company email address like David's address

Explanation: The malicious developers could have copied the email signature from John's email, if they had access to an email from him. It contains the original email address, which indicates that this is a scam.

Facts: Using a technique called "email spoofing", malicious developers can manipulate John's email address, so David will see the email as being sent from John. In this case, the malicious developers did not manipulate the email address but only the name. This is because in case David wanted to reply to the email, it would be sent to John instead of the malicious developers

Tips: If you are in doubt, reply to the email and ask a question only your boss would be able to answer