The easiest way to visualize how this works is to think of it like real postal mail. When you send a letter, you send it in an envelope. The envelope contains your message. The return address written on the postal mail is the same as the "MAIL FROM:" return address and the "RCPT TO:" will give the recipients address. This is all part of the envelope information.
The message content (inside the postal mail envelope) is like the body of your message. In the email sense it can also have the header field information such as "Subject:" "Date:" the "To:" and "From:".
The "MAIL FROM" command specifies the address for return purposes (example: issues with mail delivery).
The "From:" header field will tell you who the author of the message is.