SMTP (Simple Mail Transfer Protocol)
SMTP is the internet standard protocol for sending email between servers — the transport layer that carries a message from your application or client to the recipient's mail server.
How SMTP works
SMTP is the language mail servers speak to hand messages to one another. When you send an email, your client or application connects to an SMTP server, authenticates, and submits the message; that server then relays it toward the recipient's mail server, which stores it for retrieval.
SMTP handles sending only. Retrieving mail into an inbox is done by other protocols (IMAP or POP). Almost every email platform exposes an SMTP relay so existing apps can send through it with minimal changes.
SMTP ports and security
- Port 587 — the modern standard for authenticated submission with STARTTLS encryption
- Port 465 — submission over implicit TLS, still widely supported
- Port 25 — server-to-server relay; often blocked for client submission to curb spam
- Always authenticate and encrypt: credentials plus TLS keep the connection secure
SMTP relay vs email API
An SMTP relay is the universal option — any system that can send mail already speaks SMTP, so pointing it at a provider's relay is often a config change, not a code change. An email API offers richer control (templating, structured events, higher throughput) but requires integrating with the provider's endpoints. Many senders use SMTP for legacy apps and the API for new, high-volume flows.
Frequently asked questions
Which SMTP port should I use?
Use 587 with STARTTLS for authenticated submission — it's the current standard. 465 (implicit TLS) is a fine alternative. Avoid 25 for client submission, as it's commonly blocked.
SMTP relay or email API — which is better?
SMTP relay is the fastest way to route existing apps with no code changes. An email API gives more control and throughput for new integrations. Many teams use both.
Does SMTP handle authentication like SPF and DKIM?
No — SMTP is the transport. SPF, DKIM and DMARC are separate authentication layers applied to the message; they work alongside SMTP but aren't part of it.
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