What Is SMTP (Simple Mail Transfer Protocol)?
SMTP is the standard protocol used to send and relay email messages across the internet. Every email you send passes through SMTP servers on its way to the recipient.
How SMTP Works
When you hit send on an email, your email client connects to an SMTP server and submits the message. The SMTP server looks up the recipient's domain using DNS to find the MX record, which points to the receiving mail server. It then establishes a connection to that server and delivers the message using a series of SMTP commands (EHLO, MAIL FROM, RCPT TO, DATA).
If the receiving server is unavailable, the sending SMTP server queues the message and retries delivery for a configurable period (typically up to 5 days) before generating a bounce notification.
Why SMTP Matters
SMTP is the backbone of email delivery. Understanding how it works is essential for troubleshooting deliverability issues, configuring mail servers, and implementing email authentication protocols like SPF, DKIM, and DMARC. Misconfigured SMTP settings are one of the most common causes of email delivery failures.
Frequently Asked Questions
What port does SMTP use?
SMTP traditionally uses port 25 for server-to-server relay. Port 587 is the standard for email submission with STARTTLS encryption. Port 465 is used for implicit TLS. Most modern providers require port 587 with authentication.
Is SMTP secure?
SMTP was originally designed without encryption. Modern implementations use STARTTLS or implicit TLS to encrypt connections. Protocols like MTA-STS help enforce encrypted SMTP connections between servers.
What is the difference between SMTP and IMAP?
SMTP handles sending and relaying email, while IMAP handles retrieving and reading email from a server. Both are needed for a complete email workflow.