How the protocol works in practice: submission ports, STARTTLS, AUTH mechanisms, reply codes — and how our own SMTP server is built.
2 articles
A protocol-level walk through SMTP authentication — the EHLO handshake, STARTTLS, the PLAIN and LOGIN mechanisms byte for byte, and how to read the reply codes when a login fails — with transcripts you can reproduce with openssl and swaks.
Four ports, two TLS models, one deprecation that was later reversed. What each SMTP port is for, what a submission client should use today, and how to tell a connection problem from a TLS-mode mismatch.
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