SMTP — Simple Mail Transfer Protocol
Was ist SMTP?
Dieser Eintrag liegt derzeit nur auf Englisch vor.
SMTP (Simple Mail Transfer Protocol) is the application-layer protocol email uses to move a message from one server to the next — from an application or mailbox to the sending server, and from the sending server to the recipient's MX host. It works as a line-oriented request-response conversation: the client issues commands such as EHLO, MAIL FROM, and RCPT TO, and the server answers each with a numeric reply code before the message body transfers over the DATA phase.
More detail
Port selection tells senders how the connection is used. Port 587 is the authenticated submission port senders use to hand mail to their provider, STARTTLS upgrades the plain connection to TLS in place; port 465 does the same with implicit TLS from the first byte. Port 25 is server-to-server relay, and most cloud networks and ISPs block outbound 25 to curb spam, so new integrations should expect 587 or 465.
Server-to-server delivery is what makes sender authentication work. The receiving MX looks up the sender's SPF record and validates the DKIM signature on the message, and a sending platform's deliverability stack — DMARC policy, MTA-STS, and TLS reporting — describes how those SMTP hops treat its mail. Applications typically skip raw SMTP entirely and send through an email API, which handles queueing, retries, and bounce classification for them; the same protocol runs underneath either way.
Häufige Fragen
- Which SMTP port should a sender use?
- Port 587 with STARTTLS is the standard for authenticated submission from an application to its email provider; port 465 is the implicit-TLS equivalent. Port 25 is reserved for server-to-server relay and is blocked outbound by most cloud providers and ISPs — do not plan an integration around it.
- What is the difference between SMTP and an email API?
- SMTP is the wire protocol; an email API is a service layer over the same protocol that adds queueing, retry scheduling, bounce classification, and suppression handling. Whether an application opens an SMTP connection itself or posts to an email API, the provider's servers still relay the message to the recipient's MX host over SMTP.
- How do SPF, DKIM, and DMARC relate to SMTP?
- They ride on the SMTP dialogue. The receiving server checks the connecting IP against the sender's SPF record, validates the DKIM signature carried in the message headers, and applies the domain's DMARC policy to decide whether to accept, quarantine, or reject the message.
See also
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