SMTP

How Telescope verifies SMTP server availability, greeting health, and TLS capability.

SMTP monitors verify that a mail server accepts connections and responds correctly to the SMTP handshake sequence. Checks do not send email payloads and do not perform authentication.

How a check works

Telescope opens a connection to the configured host and port.
It reads the server greeting and validates that SMTP service is ready (220).
Telescope sends EHLO (with automatic HELO fallback for legacy servers).
It parses SMTP capabilities, including STARTTLS support when advertised.
If TLS is required, Telescope negotiates TLS using either:
- STARTTLS (explicit TLS upgrade), or
- implicit TLS (SMTPS, commonly port 465).
Telescope sends QUIT, records response time, and stores structured protocol details.

Status classification

Condition	Status	What it means
Handshake succeeds and policy checks pass	Up	SMTP endpoint is reachable and protocol flow is healthy
Connection timeout/refusal/DNS failure	Down	SMTP endpoint is unreachable
Unexpected SMTP reply codes (4xx/5xx at critical handshake stages)	Down	Server is reachable but not accepting expected SMTP flow
TLS required but no secure transport available	Down	Server does not satisfy monitor TLS policy
Greeting/banner verification fails	Down	Server identity/content check failed

SMTP monitors currently classify results as up/down (no degraded state).

Configuration options

Setting	Default	Description
`port`	`587`	SMTP endpoint port
`requireTls` / `requireStartTLS`	`false`	Require secure SMTP transport
`implicitTls`	auto (`true` on port `465`)	Use TLS immediately on connect (SMTPS)
`verifyBanner`	unset	Optional substring that must appear in server greeting
`ehloDomain`	`telescope.local`	EHLO/HELO identity sent by checker

Result data highlights

SMTP checks store structured metadata in result_data, including:

connection_mode (plain, starttls, implicit_tls)
banner
ehlo_domain
supports_starttls
tls_active, starttls_negotiated
tls_protocol, tls_cipher
extensions (advertised SMTP capabilities)
response_codes

Operational notes

Checks support both modern ESMTP (EHLO) and legacy SMTP (HELO fallback).
STARTTLS follows RFC behavior by re-issuing EHLO after TLS negotiation.
If you need certificate trust/expiry validation, pair SMTP monitors with an SSL monitor on the same host/port.