SMTP (Simple Mail Transfer Protocol)

What is SMTP?

SMTP is the long-standing protocol that governs how emails are transmitted across the internet. First defined in the early 1980s, it remains the backbone of email communication. SMTP ensures that your message leaves the sender’s server and reaches the correct receiving server based on MX Records.

Why it matters

SMTP’s simplicity and universality are why it has endured for decades. Any application or system that can talk SMTP can send email. However, because it predates modern security concerns, SMTP often requires additional layers like TLS encryption and authentication to be secure in today’s environment.

How it works

When you send an email, your application or ESP establishes a connection with an SMTP server. The server authenticates the request and passes the message through a chain of servers until it reaches the recipient's domain. That domain uses MX records to determine where the email should land. Along the way, checks like SPF, DKIM, and DMARC help verify that the sender is legitimate.

Examples

• A WordPress plugin configured with Gmail’s SMTP server to send notifications.

• An in-house application using AutoSend’s SMTP relay to send invoices.

• A developer using Outlook’s SMTP settings for testing.

Best practices

• Always use secure connections (TLS/SSL).

• Store SMTP credentials in a safe location such as an environment variable or vault.

• Consider upgrading to an Email API if you need scalability, analytics, or advanced features.

Related terms

Email API, MX Record

FAQs

Is SMTP still relevant in the age of APIs?
Yes. While APIs provide more advanced functionality, SMTP is still essential for compatibility with many systems. It remains the “lowest common denominator” for email sending and will likely remain so for years to come.

Is SMTP secure?
Not by default. Originally, SMTP was designed for a trusted network and had no security features. Today, you must use TLS encryption and authenticated connections. Without these, your messages could be intercepted or rejected.

When should I use SMTP vs an API?
If you only need to send a small number of emails or are working with a system that doesn’t support APIs, SMTP is perfectly fine. For larger volumes, analytics, or event tracking, APIs are more reliable and scalable.