Everything You Need to Know About SMTP (Developer Guide)

If you searched for everything you need to know about SMTP, start with this model:

• SMTP is the protocol for sending email
• commands and response codes define each step in delivery
• ports and TLS mode must match
• reliable sending requires testing plus SPF, DKIM, and DMARC controls

Quick answer: what is SMTP?

SMTP (Simple Mail Transfer Protocol) is the standard protocol used to submit and relay outbound email.

It is not a mailbox-reading protocol. Retrieval is handled by IMAP or POP3.

How SMTP works

1. Your app or mail client opens a TCP connection to an SMTP server.
2. The server greets with 220 and advertises capabilities after EHLO.
3. Client negotiates security (STARTTLS) and authentication (AUTH) when required.
4. Client sends envelope sender and recipients using MAIL FROM and RCPT TO.
5. Client sends message content after DATA.
6. Server accepts (250) and queues for onward delivery.

SMTP commands you should know

Command	Purpose
EHLO	Start session and discover ESMTP capabilities
HELO	Legacy greeting command
STARTTLS	Upgrade to TLS-encrypted session
AUTH	Authenticate sender identity
MAIL FROM	Declare envelope sender
RCPT TO	Add recipient
DATA	Send message headers and body
RSET	Reset transaction state
NOOP	Keepalive health check
QUIT	End SMTP session

For detailed command reference, see SMTP commands and responses.

SMTP response code classes

Class	Meaning	Typical action
2xx	Success	Continue flow
3xx	More input needed	Send next command/data
4xx	Temporary failure	Retry with backoff
5xx	Permanent failure	Fix configuration, policy, or recipient data

Common examples:

• 250 accepted
• 354 start message content
• 421 service unavailable
• 450 mailbox unavailable (temporary)
• 535 auth failed
• 550 rejected or mailbox unavailable

SMTP ports and TLS pairing

Port	Typical use	TLS mode
587	Authenticated submission	STARTTLS
465	Authenticated submission	Implicit TLS
25	Server-to-server relay	Optional/varies
2525	Alternate submission	Provider-specific

Most application senders should start with 587 + STARTTLS.

Related guides:

• SMTP port guide
• What SMTP port should I use?
• Which SMTP port should I use?

Production SMTP checklist

1. Validate command flow in Email Sandbox.
2. Add delivery assertions using Email integration testing.
3. Capture bounces and failures with Email Webhooks.
4. Enforce sender authentication with SPF, DKIM, and DMARC checks.
5. Monitor deliverability using Email deliverability testing.
6. Route retries/fallback using Email automation routing.

Common SMTP failure patterns

Auth failures (535)

Usually caused by wrong credentials, unsupported auth mechanism, or provider policy mismatch.

Recipient rejections (550)

Often caused by nonexistent mailbox, relay policy, or sender reputation/policy issues.

TLS mismatch

Using STARTTLS on implicit TLS ports (or vice versa) causes negotiation failures.

For specific rejection fixes, see Fixing SMTP 550 errors.

FAQ

Is SMTP the same as IMAP?

No. SMTP sends mail. IMAP reads/syncs mailbox state.

Can SMTP acceptance guarantee inbox placement?

No. Accepted mail can still be filtered, deferred, or placed outside primary inbox.

Do I need SMTP if I use an email API?

Not always directly, but email APIs still operate on SMTP-compatible delivery infrastructure.

Next steps

• SMTP protocol explained
• SMTP commands and responses
• What is an SMTP server?