Spoofing - (a.k.a. IP Spoofing, Fake mail, Spoofing Attack) - Spoofing is when somebody gets an e-mail that appears to come from somebody they trust, except it was actually sent maliciously, or in jest, by someone else.  IP spoofing is when someone tries to get access to somebody's computer and network by the same method.
       If a hacker can connect to a SMTP port (which they often can) then the fake e-mail message can be entered into the Internet from a backdoor (meaning not via a normal e-mail program) and it slips right into all the other e-mail messages flying around, not looking any different than the other messages. https://en.wikipedia.org/wiki/Spoofing_attack