Obviously junk, but how does a mail arrive in my inbox (with a size & date), but with the "From" and "Subject" lines totally clear?!?

 

easy. The info stored in the sections you stated is just data carried in the messgage body.
If you a little bit of how to use the SMTP protocol you can alter this information to what you want.
e.g.

$ netcat mail.isp.com 25
HELO gedi.com
MAIL FROM: The_Boogy_Man@yourbedroom.com
RCPT TO: user@isp.com
DATA
I'm going to get you!!
.

If you check the headers of the mail, you will see where it has originated from. You can hide all the information you see from within the headers by constructing the mail a little more carefully than I have detailed here.