# What is SMTP? Definition and examples · Leadraft URL: https://leadraft.co/glossary/smtp Summary: SMTP (Simple Mail Transfer Protocol) is the standard, defined in RFC 5321, that mail servers use to send and relay email to each other. Glossary · Technical · Last updated October 31, 2026 # SMTP SMTP (Simple Mail Transfer Protocol) is the standard, defined in RFC 5321, that mail servers use to send and relay email to each other. **Also called:** Simple Mail Transfer Protocol. An SMTP session is a short text conversation. The client greets the server (`EHLO`), names the sender (`MAIL FROM`), names each recipient (`RCPT TO`), sends the message after `DATA`, and quits. The server answers each step with a three-digit reply code. Reply codes are grouped by first digit: `2xx` success (for example `250`), `4xx` temporary failure (try again later), `5xx` permanent failure (for example `550` for no such user). Enhanced status codes from RFC 3463, such as `5.1.1`, add detail. Email verification uses the first half of this conversation. A verifier sends `RCPT TO` for the address, reads the reply, and quits before `DATA`, so no message is delivered. The `VRFY` command looks like it was made for this, but RFC 5321 lets servers decline to answer it and most do. Knowing the codes helps you read bounce reports. `5.1.x` codes mean bad addresses (a list problem); `5.7.x` codes usually mean policy or authentication (a setup or reputation problem); `4xx` codes mean wait and retry, not delete. ## Example `RCPT TO:` gets `250 OK`; `RCPT TO:` gets `550 5.1.1 User unknown`. The first mailbox exists; the second doesn't, and the domain isn't catch-all. ## Sources 1. 1.[IETF, RFC 5321: Simple Mail Transfer Protocol](https://www.rfc-editor.org/rfc/rfc5321) 2. 2.[IETF, RFC 3463: Enhanced Mail System Status Codes](https://www.rfc-editor.org/rfc/rfc3463) ## Related terms - [Email verificationEmail verification is checking whether an address can receive mail, using syntax, MX and SMTP tests, without sending a message to it.](https://leadraft.co/glossary/email-verification) - [MX recordAn MX record is a DNS record that names the mail servers accepting email for a domain, with a priority number for each server.](https://leadraft.co/glossary/mx-record) - [Hard bounceA hard bounce is a permanent delivery failure, such as an address that doesn't exist, returned with an SMTP reply code starting with 5.](https://leadraft.co/glossary/hard-bounce) - [Soft bounceA soft bounce is a temporary delivery failure, such as a full mailbox or busy server, returned with an SMTP reply code starting with 4.](https://leadraft.co/glossary/soft-bounce) ## Further reading - [How email verification works](https://leadraft.co/blog/how-email-verification-works) ## Every business on the map. Every email that works. 500 businesses and 50 checked emails a month, free. No card. [Start free](https://app.leadraft.co/sign-up) Last built: 2026-11-09