Messages exchanged across networks are passed between mail servers, including any attached data files (such as images, multimedia, or documents).
The transmission details are specified by the Simple Mail Transfer Protocol (SMTP).
Upon final delivery, the Return-Path field is added to the envelope to record the return path.
Common protocols for this are: Submission of new email from a mail client is via SMTP, typically on port 587 or 465, and is now generally restricted to servers the user has an account with-such as their ISP.
This is for policy, not technical, reasons so that providers have some means of holding their users accountable for the generation of spam and other forms of email abuse.