What is SMTP?

Simple Mail Transfer Protocol (SMTP) is the standard protocol used to send and relay email across the internet. A client or mail server hands a message to SMTP, which routes it hop by hop to the recipient’s mail server. Versa Director, Analytics clusters, and aggregator nodes use SMTP to send alarm notifications, reports, and registration codes.

Key takeaways

• SMTP moves email from sender to recipient mail server, hop by hop.
• ESMTP extensions add authentication, size limits, and TLS support.
• StartTLS and SMTPS encrypt SMTP sessions between mail servers.
• Versa Director and Analytics use SMTP to email alerts and reports.
• Versa’s SMTP proxy also inspects email for DLP and threat protection.

On this page: Why it matters · How it works · Types · SMTP & Versa · FAQ

Why is SMTP important?

SMTP is the protocol that makes email interoperable. Any mail server that speaks SMTP can exchange messages with any other, regardless of vendor – the same open-standard property that lets a Gmail user email an Outlook user without a custom integration. That universality is why SMTP has remained the backbone of internet email delivery since the 1980s, even as the applications built on top of it have changed completely.

For infrastructure teams, SMTP is also the default channel for automated notifications. Monitoring and network management platforms almost always use it to email alerts, because it requires no proprietary client and reaches any inbox. A correctly configured SMTP relay – reachable, authenticated, and encrypted – is a quiet but essential dependency: if it breaks, alarm notifications and password delivery go with it.

What is SMTP?

SMTP defines how one mail server hands a message to another. The sending server opens a connection, issues commands identifying itself and the sender and recipient addresses, then transmits the message body. The receiving server accepts, queues, or rejects the message and returns a status code. A message often crosses several servers this way before reaching the recipient’s mailbox, where a separate retrieval protocol such as IMAP hands it to the reader.

Plain SMTP, as defined in the original standard, has been extended by ESMTP (Extended SMTP), which mail servers negotiate automatically. ESMTP adds authentication, larger message sizes, and the StartTLS command that upgrades a plaintext connection to an encrypted one mid-session.

Types of SMTP

SMTP sessions run at different security levels. Plain text sends commands and content unencrypted – acceptable only on trusted internal links. StartTLS begins in plain text and then upgrades the connection to TLS before any credentials or content are sent, commonly on port 587 for submission. SMTPS wraps the entire session in TLS from the first byte, typically on port 465.

SMTP servers also play distinct roles. A mail submission agent (MSA) accepts outgoing mail from authenticated clients. A mail transfer agent (MTA) relays mail between servers across the internet. A mail delivery agent (MDA) places incoming mail into a recipient’s mailbox. A single server commonly performs all three roles, but large deployments – including Versa’s SMTP proxy – separate them so inspection happens at a distinct hop from delivery.

SMTP & Versa

Versa Director, Analytics clusters, and aggregator nodes are configured with one or more SMTP servers to send email: alarm notifications, scheduled reports, registration codes, and other system alerts. SMTP notification configurations can be set at the system level or per tenant, with hostname, SSL port, credentials, and From/To addresses; tenant configurations can be pushed to child tenants, and a Send Test Email action confirms one works. Separately, SMTP server settings under Users/Groups support two-factor authentication for Versa Secure Access, emailing a one-time password with a choice of Plain text, SMTPS, or StartTLS security levels.

Versa also uses SMTP defensively. Email protection in Concerto places a Versa SMTP proxy between a cloud email exchange – Gmail, Microsoft 365, or a custom service – and its upstream mail transfer agent. Inbound and outbound messages pass through the proxy, which enforces Data Loss Prevention and Advanced Threat Protection policies, forwarding, redacting, quarantining, or dropping mail before delivery, with TLS enforced throughout.

Frequently asked questions

What port does SMTP use?

Port 25 is used for server-to-server relaying, port 587 for authenticated client submission (typically with StartTLS), and port 465 for SMTPS, which wraps the entire session in TLS from the start. Modern mail clients almost always submit outgoing mail on port 587, not port 25.

What is the difference between SMTP and ESMTP?

ESMTP is SMTP extended with capabilities the original 1982 standard lacked, including authentication, larger message sizes, and StartTLS encryption. Servers negotiate ESMTP automatically at connection time, so in practice nearly all SMTP traffic seen on the internet today is actually ESMTP.

What is the difference between SMTP and IMAP or POP3?

SMTP sends and relays mail between servers; it is a push protocol. IMAP and POP3 are retrieval protocols a mail client uses to pull messages from a mailbox after SMTP has delivered them. A single email typically uses SMTP on the way in and IMAP or POP3 on the way out to the reader.

Is SMTP encrypted?

Not by default. Plain SMTP sends commands and message content in clear text, fully unencrypted end to end. StartTLS and SMTPS add encryption, and most modern mail servers now require one of them before accepting a connection, refusing plaintext relaying from unfamiliar senders.

Why does Versa Director need an SMTP server configured?

Director, Analytics clusters, and aggregator nodes rely on SMTP to deliver alarm notifications, scheduled reports, and registration codes by email. Without a working SMTP configuration, those alerts have no delivery path, and administrators lose visibility into system events until it is fixed.

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