I used to spoof emails to my teachers in high school asking them to come to the principle's office asap, and email the principle from another random teacher at the same time for him to come to the class room, and that random teacher to expect the principal at a certain time. We'd be teacher free for quite awhile because the principal wasn't there, and our teacher was.
Sure, but my point was different: let's say one of your teacher answered the spoofed email from the principal, you wouldn't be able to (properly) answer that email since you wouldn't receive it. So, in the case of an email exchange between two people, one can't claim his/her emails were spoofed, as the spoofer wouldn't be able to answer the other party's emails in a precise and on topic way. This is without even considering that, bh default, most email clients include the previous messages inside a reply. Meaning that the spoofer would somehow be able to know the other party's reply exactly word by word.
You don't need a conversation to cause havoc.