Definition
SMTP (Simple Mail Transfer Protocol) settings are the configuration details that enable an email client, application, or server to send email through an SMTP server. The standard settings include the SMTP server hostname, port number, authentication credentials, and encryption requirements (TLS or SSL).
While most email marketers never configure SMTP settings directly (their ESP handles this), understanding SMTP configuration is important for troubleshooting deliverability issues, setting up custom sending infrastructure, and communicating with development teams about email system requirements.
Common SMTP Configuration
| Setting | Common Values | Purpose |
|---|---|---|
| SMTP host | smtp.example.com | The mail server address |
| Port | 25, 587, 465, 2525 | Connection endpoint |
| Encryption | STARTTLS, SSL/TLS | Secure transmission |
| Username | Your email address | Authentication |
| Password | Your email password | Authentication |
| Authentication | Required (most servers) | Verifies identity |
SMTP Ports Comparison
| Port | Encryption | Use Case |
|---|---|---|
| 25 | None (historically) | Legacy, often blocked by ISPs |
| 587 | STARTTLS | Standard submission port |
| 465 | SSL/TLS | Legacy SMTPS, still widely used |
| 2525 | Various | Alternative if port 587 is blocked |
Best Practices
- Use port 587 with STARTTLS as your primary SMTP configuration
- Never use port 25 for email submission — it is frequently blocked by ISPs and cloud providers
- Store SMTP credentials securely — never hardcode them in application code
- Test SMTP configuration after any server or DNS change
- Use dedicated SMTP credentials for different applications or services
- Monitor SMTP connection logs for authentication failures and delivery issues
- Consider using an email API service instead of direct SMTP for higher-volume sending
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Related Glossary Terms
IP Pool
An IP pool is a group of dedicated IP addresses that a sender uses collectively to distribute sending volume and manage reputation, providing flexibility for different types of email traffic and volume spikes.
Email Sending Domain Strategy
The practice of using multiple sending subdomains for different email types to isolate reputation, protect deliverability, and improve inbox placement.
Email Sending Reputation Monitoring
The ongoing process of tracking sender score, domain reputation, IP reputation, and feedback loop data across ISPs to detect and respond to deliverability changes.
Email Spam Filter Tuning
The process of adjusting email content, sending patterns, and infrastructure to improve inbox placement by adapting to current spam filter algorithms.
Submission Rate
Submission rate is the percentage of emails submitted for delivery that are accepted by the email service provider or sending infrastructure, versus those rejected due to formatting errors, policy violations, or rate limits.
Frequently Asked Questions
SMTP is a protocol for sending email — it is the underlying technology. An email API (like SendGrid or Mailgun's API) is a developer-friendly wrapper around SMTP that provides better deliverability, analytics, and reliability. For any programme sending more than a few thousand emails per month, an email API is recommended over direct SMTP configuration.
Common issues include incorrect server hostname, wrong port (port 25 being blocked), missing or incorrect authentication credentials, TLS/SSL mismatch, and firewall rules blocking outbound SMTP connections. Verify each setting individually and test with a simple SMTP test tool to isolate the issue.