In simple terms
MAIL FROM – Mr.Blue used XYZ post office to send a mail envelope to Mr.Red, and if Mr.Red’s post office cant locate Mr. Red then the envelope can be sent back to XYZ Post office to be returned to Mr.Blue
FROM – This message is FROM Mr.Blue to Mr. Red (Receiver).
Now the technical stuff:
Like the mail envelope which contains the message, An email envelope also contains a message.
- The email envelope will have three pieces of identification information, the host greeting, the “MAIL FROM:” return address and the “RCPT TO:” list of recipient addresses.
- The message content comprises a set of header fields and a body. The body, in turn can be simple text or can be a structured, multi-media “MIME” object of attachments. The set of header fields can be quite extensive, but typically at least include: “Subject:” “Date:” the “To:” and “From:”.
The “MAIL FROM” command specifies the address of the recipient for return notices if any problems occur with the delivery of the message, such as non-delivery notices.
The “From:” header field indicates who is the author of the message.
The technical notation for referring to components of email information is: RFC5321.MailFrom and RFC5322.From according to the IETF RFCs where the field is defined and the specific field being referenced.