SMTP Explained: How It Works and What to Look For
SMTP transfers email between mail clients and servers or between mail servers.
Published by Protocol Explorer · Reviewed
How SMTP works
An SMTP session uses commands such as EHLO, MAIL FROM, RCPT TO and DATA, with server reply codes indicating the outcome of each stage.
What to look for in an exchange
Envelope sender and recipient addresses guide delivery and are distinct from message headers. STARTTLS can upgrade a supported SMTP session to TLS.
Follow an example step by step
- Read the server greeting and the client EHLO command.
- Follow MAIL FROM and RCPT TO to identify the message envelope.
- Inspect DATA and the subsequent server response to see whether that server accepted responsibility for the message.
Common interpretation mistake
An SMTP acceptance response does not guarantee final delivery to the recipient inbox.
Key point to remember
Message headers such as From and To are distinct from the SMTP envelope.
Explore the protocol
The SMTP overview is free without an account. Its full interactive lesson is included in Protocol Explorer Pro, a one-time $4.99 purchase; an account is required to buy Pro and restore the licence across devices.
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Specification
Read the primary specification for SMTP for the authoritative protocol details. The interactive scenario is a simplified teaching model, not a capture of your network.
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