SMTP (Simple Mail Transfer Protocol)

What Is SMTP?

SMTP (Simple Mail Transfer Protocol) is the standard internet protocol for sending email between servers. When you click "Send" in your email client, SMTP is the protocol that carries your message from your mail client to your mail server, from your mail server to the recipient's mail server, and between any intermediate relays along the way. Almost all email — whether sent from Gmail, Outlook, a corporate mail server, or an ESP like Mailchimp — ultimately uses SMTP for server-to-server communication.

How SMTP Works

SMTP is a command-response protocol. The sending server initiates a TCP connection to the receiving server (port 25 for server-to-server, port 587 for client-to-server submission). They exchange a greeting sequence (EHLO/HELO). The sender specifies the envelope from address (MAIL FROM). The sender specifies the recipient (RCPT TO). The receiver either accepts or rejects the recipient. If accepted, the sender transmits the message (DATA). The receiver acknowledges receipt. Connection closes. This handshake is the foundation of all email delivery.

SMTP and Email Verification

SMTP verification — the technique at the core of EmailVerify.io's mailbox-level check — exploits this handshake sequence. The verifier connects to the recipient's mail server, sends EHLO and MAIL FROM, then sends the RCPT TO command with the address being verified. A 250 response (accepted) indicates the mailbox exists. A 550 response (rejected) indicates it doesn't. The verifier then sends QUIT without actually transmitting a message. This probes mailbox existence without sending any email.

SMTP Error Codes

SMTP uses numeric codes to communicate status. 2xx codes indicate success. 4xx codes indicate temporary failures (soft bounces) — the server is saying "try again later." 5xx codes indicate permanent failures (hard bounces) — the server is permanently rejecting the message. The most important email codes: 250 (OK, accepted), 421 (service unavailable — soft bounce), 450/451/452 (temporary failure — soft bounce), 550 (mailbox unavailable — hard bounce), 553/554 (policy rejection — hard bounce).

SMTP Authentication (SMTP AUTH)

For outgoing email, most ESPs and mail servers require SMTP authentication — you must provide credentials (username and password) to prove you're authorized to send through that server. This is different from the authentication protocols (SPF/DKIM/DMARC) that receiving servers use to evaluate incoming mail. SMTP AUTH prevents your mail server from being used as an open relay by unauthorized senders.

SMTP Ports

Port 25: server-to-server SMTP (often blocked by ISPs for residential connections to prevent spam from compromised home computers). Port 587: the standard submission port for clients sending to their outgoing mail server — should always use STARTTLS encryption. Port 465: SMTP over implicit TLS (older standard, still widely supported). Most ESP sending uses port 587 with TLS. Understanding SMTP ports matters when configuring mail servers or debugging connection issues. For reliable email delivery, use EmailVerify.io to ensure your list is clean before any SMTP-based sending. Start at emailverify.io.