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esp-idf-smtp

Lightweight SMTP client for ESP32 devices using esp_tls.

Supports implicit TLS, STARTTLS, and plaintext connections with AUTH PLAIN/LOGIN authentication. Builder pattern for email composition. Host-testable protocol logic via transport trait abstraction.

Features

Three TLS modes: Implicit TLS (port 465), STARTTLS (port 587), Plain (port 25)
Authentication: AUTH PLAIN and AUTH LOGIN, auto-selected from server capabilities
Multiple recipients: To, CC, BCC with correct header generation
RFC compliant: Dot-stuffing (RFC 5321), CRLF line endings, EHLO capability parsing
Type-safe errors: Every failure mode has a typed error variant
Host-testable: Protocol logic runs on any platform via SmtpTransport trait
Feature-gated: ESP-IDF transport behind esp-idf feature flag

Quick Start

use esp_idf_smtp::{SmtpConfig, Email, TlsMode, SmtpClient};

let config = SmtpConfig::new("smtp.gmail.com", 465)
    .tls_mode(TlsMode::ImplicitTls)
    .credentials("user@gmail.com", "app-password")
    .timeout_ms(10_000);

let email = Email::builder()
    .from("device@example.com")
    .to("alert@example.com")
    .subject("Nightwatch Alert")
    .body("Safety monitor triggered an alert.")
    .build()?;

SmtpClient::new(config).send(&email)?;

Feature Flags

Feature	Default	Description
`esp-idf`	Yes	ESP-IDF transport via `esp_tls`. Adds `esp-idf-svc` dependency.

Without esp-idf, only protocol types and the SmtpTransport trait are available — useful for host testing or providing your own transport.

# Host-only (no ESP-IDF)
[dependencies]
esp-idf-smtp = { version = "0.1", default-features = false }

TLS Modes

use esp_idf_smtp::{SmtpConfig, TlsMode};

// Gmail (implicit TLS on 465)
let config = SmtpConfig::new("smtp.gmail.com", 465);

// Corporate relay (STARTTLS on 587)
let config = SmtpConfig::new("mail.corp.com", 587)
    .tls_mode(TlsMode::StartTls);

// Local test server (no TLS)
let config = SmtpConfig::new("localhost", 25)
    .tls_mode(TlsMode::Plain);

// Self-signed server
let config = SmtpConfig::new("mail.local", 465)
    .skip_cert_verification();

Multiple Recipients

let email = Email::builder()
    .from("device@example.com")
    .to("ops@example.com")
    .to("oncall@example.com")
    .cc("manager@example.com")
    .bcc("audit@example.com")
    .subject("Alert")
    .body("Details here.")
    .build()?;

BCC recipients receive the email but are not included in message headers.

Host Testing

The protocol engine operates against the SmtpTransport trait. Implement it with a mock to test SMTP logic without hardware:

cargo test --no-default-features

Minimum Supported Rust Version

1.75

License

This project is licensed under the Mozilla Public License 2.0 — see LICENSE for details.

You can use this library in closed-source projects. If you modify any of the source files in this library, the modified files must be made available under the MPL-2.0 when distributed.

esp-idf-smtp 0.4.0