What Is SMTP (Simple Mail Transfer Protocol)?

What is SMTP (Simple Mail Transfer Protocol)?

SMTP (Simple Mail Transfer Protocol) is the standard protocol used to send, relay, and route email messages between mail servers. It defines how email is transmitted across the internet, ensuring that messages travel from the sender’s mail server to the recipient’s mail server reliably and securely. SMTP operates primarily on port 25, though ports 465 and 587 are also used for encrypted and authenticated delivery.

Originally defined in RFC 5321, SMTP forms the foundation of the global email infrastructure. Every time a message is sent, it is wrapped in SMTP commands and delivered through a sequence of handshakes between sending and receiving systems. This protocol works closely with others, such as IMAP and POP3 to enable full email communication, with SMTP handling outgoing mail and the others managing message retrieval.

How SMTP Works

SMTP communication follows a client-server model, where the sender’s system acts as the client and the recipient’s system as the server. When a user sends an email, the mail client (MUA) submits the message to an outgoing mail server (MTA), which delivers it through a series of SMTP relays until it reaches the destination domain.

The process typically involves the following steps:

• Connection: The sending MTA connects to the recipient’s mail server on port 25 or 587.
• Handshake: The client identifies itself with the HELO or EHLO command.
• Mail Transaction: The sender address (MAIL FROM) and recipient address (RCPT TO) are provided.
• Data Transfer: The message body and headers are transmitted using the DATA command.
• Delivery Confirmation: The receiving server replies with a success (2xx) or error (4xx/5xx) status code.

After successful delivery, the recipient’s server stores the message in a mailbox, where it can be accessed using IMAP or POP3. If the destination server is unreachable, the sending server retries delivery at scheduled intervals before generating an NDR (Non-Delivery Report).

SMTP Security and Authentication

Modern SMTP implementations include authentication and encryption to prevent abuse and eavesdropping. Without these protections, spammers or attackers could exploit open relays to send malicious or spoofed emails.

Key security mechanisms include:

• STARTTLS: Upgrades plain SMTP connections to encrypted sessions using TLS
• SMTP AUTH: Requires valid credentials before sending mail
• SPF: Verifies that the sending server is authorized for the domain
• DKIM: Adds cryptographic signatures to verify message integrity
• DMARC: Aligns authentication results with the visible From domain to enforce policy

Together, these technologies make SMTP communications more trustworthy and resilient against spoofing, phishing, and man-in-the-middle attacks.

SMTP and DMARCeye

DMARCeye monitors and analyzes the SMTP flow behind every message to detect authentication failures, misconfigurations, and spoofing attempts. By examining SPF, DKIM, and DMARC results within SMTP transactions, the platform provides clear visibility into which servers are legitimately sending mail for your domain and which are not.

DMARCeye’s analytics also highlight inconsistencies in envelope senders, relay behavior, or unauthorized SMTP hosts attempting to impersonate your domain. These insights help organizations maintain compliance, improve deliverability, and strengthen their overall email security posture.

Sign up for a free trial of DMARCeye today and secure your email domain.

To learn more about DMARC and DMARC-related terms, explore the DMARCeye Glossary.