How SMTP works?

The software on the user's side - Mail User Agent (MUA) connects via SMTP to the server's Mail Submission Agent (MSA) via ports 25 or 587.

Then Mail Transfer Agent (MTA) transmits data via Internet to the recipient mail server's Mail Delivery Agent (MDA).

Once email has been delivered to the destination mail server, recipient connects to the mail server and downloads email messages via protocols POP or IMAP.