Infrastructure
What is Simple Mail Transfer Protocol (SMTP)?
The internet standard protocol for sending and relaying email between servers.
SMTP is the protocol mail clients and servers use to submit and transfer outbound email across the internet. It governs the handshake and the envelope (MAIL FROM and RCPT TO) and hands the message off, typically over port 587 for submission or 465 for implicit TLS. SMTP handles transmission only; retrieving mail from a mailbox uses separate protocols like IMAP or POP3.
Why it matters for cold email
Cold-email tools connect to mailboxes over SMTP to send, so credentials, port, and TLS settings directly determine whether mail goes out and how it authenticates. The SMTP envelope's MAIL FROM domain is what SPF checks, and a misconfigured relay can break alignment or get the connection throttled, capping safe sending volume.
Related terms
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