Email "submission" and "relay" event

Solution 1:

Scenario 2 is close if "sending" is stated as the initial event, authenticated or authorized, of handing mail to the distribution system of email, handled by mail-transfer-agents. Submission can only happen once during the lifecycle of mail delivery - at the beginning. Any other smtp to smtp transfer between MTAs is named relaying.

The submission split from the ongoings of port 25 is currently defined in rfc 6409. Reading its introduction explains the reason why unrestricted mail relaying needed an additional concept of "authenticated/authorized" sending, thus "submitting" of mail.

So to reiterate a common lifecycle of an email nowadays is

client (mail-user-agent) => submission (mail-submission-agent, part of the smtpd) => relay*(n) => delivery

If you configure a postfix and even use a traditional mail program as mutt (check their MailConcept page), you see how it's very much a lot of small services combined at every step.