SMTP is the transport system behind Internet email. It moves a message from one system to another; it does not define how a user reads a mailbox.
A normal SMTP conversation
A basic server-to-server exchange looks roughly like this:
S: 220 mx.example.net ESMTP
C: EHLO sender.example
S: 250-mx.example.net
S: 250-STARTTLS
C: STARTTLS
...
C: MAIL FROM:<bounce@example.org>
S: 250 2.1.0 OK
C: RCPT TO:<user@example.net>
S: 250 2.1.5 OK
C: DATA
S: 354 End data with <CRLF>.<CRLF>
...
S: 250 2.0.0 queued
C: QUITReal sessions can advertise many more ESMTP extensions, negotiate TLS, authenticate a submitting client, reject individual recipients, or temporarily defer delivery.
Submission vs transfer
Port 25 is primarily used between mail servers. End-user and application submission normally uses 587, or 465 with implicit TLS. Keeping submission separate from server-to-server transfer lets operators apply authentication and relay policy without breaking public mail exchange.
The envelope matters
MAIL FROM and RCPT TO form the SMTP envelope. They are not the same thing as the visible From: and To: headers inside the message. This distinction explains a lot of behavior around Bcc, bounces, SPF and DMARC.
Replies
A 2xx reply is successful, 4xx is normally temporary, and 5xx is normally a permanent failure for that attempt. Keep the full remote reply when troubleshooting; the text and enhanced status code often tell you more than the first three digits.