Email is insecure: But there nothing on web explaining it

I just had a chat with friend of mine explaining his how DKIM and SPF technologies are just for spam detections so the best way (the only way?) is to use something like PGP for secure email messaging. Or to use project messaging application.

The I said "let me google that for you", and.. nothing. There is no single article explain it for normal users why and how email is insecure (i.e., for example, that sender is really the one who says it is).

So are they any good articles on this topic?