Glossary · Protocols
SMTP is the protocol that moves email between servers, and from your mail app to your provider when you send.
Defined in RFC 5321, SMTP carries every message across the internet. Servers deliver to each other on port 25. Mail apps submit outgoing mail to their provider on port 465 (TLS from the start) or 587 (upgraded with STARTTLS), after logging in.
SMTP doesn't verify who the sender is; anyone can claim any address. That's why SPF, DKIM and DMARC were layered on top.
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