It's really more like mail in an envelope -- it's illegal to intercept, and generally private, but is pretty easy to get at in transit if you put your mind to it.
Lots of mailservers do opportunistic encryption of the SMTP channel these days, too, so you can't eavesdrop as a passive attacker anymore.
Now you have to ~intercept the envelope, open it, read it, put the mail in a new envelope, and send it on the the destination~.