Definition
An SMTP server (also called a mail transfer agent or MTA) is a computer application that sends, receives, and relays email using the Simple Mail Transfer Protocol. When you send an email, your email client or application submits it to an SMTP server, which routes it to the recipient's mail server.
SMTP servers are the core infrastructure of email delivery. Every email passes through at least one SMTP server on its journey from sender to recipient.
SMTP Server vs SMTP Relay
| Component | Role | Example |
|---|---|---|
| SMTP Server | Accepts, routes, and delivers email | Postfix, Exim, Microsoft Exchange |
| SMTP Relay | Forwards email from one server to another | SendGrid, Mailgun, AWS SES |
Not all SMTP servers are relays. A relay specifically forwards email from one server to another. A server can accept email for local delivery without relaying it.
SMTP Server Functions
| Function | Description |
|---|---|
| Accept incoming mail | Receive email from senders |
| Queue outgoing mail | Hold messages for delivery |
| Route to destination | Look up MX records and connect to receiving server |
| Handle bounces | Return undeliverable messages |
| Authentication | Verify authorised senders (SMTP AUTH) |
| Rate limiting | Control sending velocity |
SMTP Server Software
| Software | Platform | Best For |
|---|---|---|
| Postfix | Linux/Unix | High-performance sending |
| Exim | Linux/Unix | Flexibility, configuration |
| Sendmail | Linux/Unix | Legacy systems |
| Microsoft Exchange | Windows | Enterprise email |
| Haraka | Node.js | Custom, high-performance |
SMTP Server Configuration
Key configuration parameters for email marketing:
| Parameter | Impact | Setting |
|---|---|---|
| Queue size | How many emails waiting to send | Monitor regularly |
| Retry schedule | How often to retry failed sends | 30min, 1hr, 4hr intervals |
| Rate limiting | Max sends per second | Set below ISP limits |
| Timeout | How long before giving up on a connection | 30-60 seconds |
| TLS requirement | Whether encryption is required | Required for modern delivery |
SMTP Server Best Practices
- Use authenticated SMTP: Require SMTP AUTH for all outgoing connections to prevent unauthorised relay.
- Configure proper reverse DNS: PTR records matching your sending domain improve deliverability.
- Monitor queue depth: Growing queues indicate delivery problems that need investigation.
- Set appropriate rate limits: Rate limits that are too high trigger ISP throttling. Limits that are too slow delay delivery.
- Implement bounce processing: Configure the server to handle bounce messages and feed them back to your email platform.
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Related Glossary Terms
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Frequently Asked Questions
Most email marketers do not need their own SMTP server. Email Service Providers (ESPs) manage SMTP infrastructure for you. Running your own server is appropriate for very high volume senders or organisations requiring full infrastructure control.
Port 25 is the default SMTP port but is often blocked by ISPs to prevent spam. Port 587 is the recommended submission port with STARTTLS encryption. Port 465 uses SSL/TLS from connection start.
The sending SMTP server looks up the recipient's domain MX record in DNS, connects to the receiving servers SMTP port, and transfers the message using SMTP commands (HELO, MAIL FROM, RCPT TO, DATA).
Overloaded servers delay delivery, increase bounce rates, and may be blacklisted. Symptoms include growing queue depth, slow SMTP response times, and connection timeouts.
When a receiving server rejects a message, the sending SMTP server generates a bounce message (NDR) that is returned to the sender. The bounce includes an SMTP status code and explanation.