smtp-test-tool

Cross-platform SMTP / IMAP / POP3 connectivity tester with IT-actionable diagnostics. CLI and GUI in one single static binary per OS, no external runtime, no OpenSSL on the host.

When your mail flow breaks at 09:00 on a Monday, this is the tool you hand to your IT department alongside an exact reproduction of the error the server returned — not "it doesn't work".

Screenshots

The GUI follows the OS appearance, with a manual auto / dark / light override on the Advanced tab. Both palettes meet WCAG 2.2 Level AAA contrast on the elements where colour carries information.

Dark	Light

Languages

The GUI, the CLI prompts, and every diagnostic hint can be displayed in any of the currently-shipped locales. The application auto- detects the OS locale on startup and applies it if a translation exists, otherwise falls back to English. The Advanced tab carries a language picker limited to your OS locale + English — deliberately two options at most, so the UI does not grow with the translation set.

Shipped in v0.1.4 — 29 languages (4 new non-Latin scripts):

Latin / Cyrillic / Greek locales render with eframe's bundled Inter/Hack fonts. The four non-Latin locales (zh, ja, ko, ar) need a system-installed font that covers their script — modern Windows / macOS / Linux desktops ship one out of the box (Microsoft YaHei, Yu Gothic UI, Malgun Gothic, Segoe UI Arabic, the Noto family on Linux). At startup the GUI consults fontdb to find one and appends it to egui's fallback chain; nothing is bundled into the binary. If your distro lacks the relevant Noto package, glyphs will render as tofu — install fonts-noto-cjk / fonts-noto-arabic to fix. Indic and Thai locales will land in a future release once their translations are commissioned.

See CONTRIBUTING.md § Translations for the native-review recipe — PRs welcome.

Features

• Three protocols in one tool: SMTP (via lettre), IMAP and POP3 (hand-rolled over rustls so we own the full wire trace).
• IT-actionable diagnostics: every server response is parsed and enriched with a human explanation. Microsoft 365's most painful codes (5.7.60 SendAsDenied, 5.7.139 Basic-Auth-disabled, 5.7.57 unauthenticated MAIL FROM, 5.7.708 IP-reputation block, …) are translated to "what to ask IT to change".
• Provider presets for the eleven mail services people actually use: Outlook.com / Hotmail, Microsoft 365, Gmail / Google Workspace, Yahoo Mail, iCloud / Apple Mail, Proton Mail (Bridge), Fastmail, Zoho Mail, AOL Mail, GMX / Mail.com, and Yandex Mail — pick one from the Provider preset menu and the SMTP / IMAP / POP3 host, port, and security fields fill themselves in. Each preset carries a small note about app-password or Bridge requirements where they apply.
• Profiles in a human-readable TOML file (smtp_test_tool.toml) auto-loaded from the executable's directory, so "verify the last-known-good settings still work" is one click.
• Accessibility is the baseline, not the goal. The GUI follows the OS dark/light setting on Windows, macOS, and Linux; colour is never the only signal (every [ PASS ] / [ FAIL ] is also textual); contrast ≥ 7:1 (WCAG 2.2 Level AAA); AccessKit is enabled for screen-reader support.
• One binary per OS, no installer required.

Install

Download a prebuilt binary

Grab the latest release for your OS from GitHub Releases.

Each archive contains both smtp-test-tool (CLI) and smtp-test-tool-gui (GUI).

With Cargo

cargo install smtp-test-tool          # CLI + GUI
cargo install smtp-test-tool --no-default-features  # CLI only

Usage

CLI

# First run with built-in Outlook.com defaults
smtp-test-tool --user me@outlook.com

# Write a starter config file next to the binary
smtp-test-tool init

# Use a saved profile
smtp-test-tool --profile production

# List profiles in the loaded config file
smtp-test-tool profiles

# Verbose diagnostic trace
smtp-test-tool --log-level debug

The exit code is 0 if every enabled protocol passes, 1 if any fail, 2 on an internal/configuration error — handy for monitoring and cron.

GUI

Double-click smtp-test-tool-gui (or smtp-test-tool-gui.exe). The form pre-fills with Outlook.com defaults; any smtp_test_tool.toml next to the binary is loaded automatically.

Config file (smtp_test_tool.toml)

active = "default"

[profiles.default]
user = "me@example.com"
smtp_host = "smtp-mail.outlook.com"
smtp_port = 587
smtp_security = "starttls"
imap_host = "outlook.office365.com"
imap_port = 993
imap_security = "ssl"
pop_host = "outlook.office365.com"
pop_port = 995
pop_security = "ssl"
pop_enabled = false

[profiles.on-prem]
user = "svc-monitor@corp.local"
smtp_host = "mail.corp.local"
smtp_port = 25
smtp_security = "starttls"
imap_host = "mail.corp.local"
imap_port = 143
imap_security = "starttls"
ca_file = "/etc/ssl/corp-internal-ca.pem"

Passwords and OAuth tokens are never written to the config file. They live in memory for the current session, or - on opt-in - in the native OS keychain:

• The GUI's Servers tab has a Save password to keychain / Forget keychain entry button pair under the credentials block.
• The CLI exposes --keychain-load (look up at startup) and --keychain-save (write after a successful test), plus a keychain status|forget subcommand.
• Backed by Windows Credential Manager, macOS Keychain, or Linux Secret Service via gnome-keyring / KWallet. Real at-rest encryption, gated by the OS unlock prompt - unlike base64.

Say cargo build --no-default-features --features gui to ship a build without the keychain code (skips the keyring crate and, on Linux, the libdbus-1-dev runtime dependency).

Example diagnostic output

2026-05-19T08:04:11Z  INFO smtp  | SMTP target smtp.office365.com:587 (starttls)
2026-05-19T08:04:11Z  INFO smtp  | TCP connection established
2026-05-19T08:04:11Z  INFO smtp  | STARTTLS negotiated, TLSv1.3
2026-05-19T08:04:12Z  ERROR smtp | SMTP AUTH FAILED
2026-05-19T08:04:12Z  ERROR smtp |   Server replied 535: 5.7.139 Authentication unsuccessful, basic authentication is disabled
2026-05-19T08:04:12Z  ERROR smtp |     ESC 5.7.139: Authentication unsuccessful, the request did not meet the criteria.
2026-05-19T08:04:12Z  ERROR smtp |     -> Action: Conditional Access policy denied the login (location, device, MFA).

That second-to-last line is what you forward to IT.

Building from source

Requires Rust 1.92 or newer (stable). The egui ecosystem sets this floor; building CLI-only with --no-default-features would in practice work on slightly older toolchains.

git clone https://github.com/Struis112/smtp-test-tool
cd smtp-test-tool
cargo build --release
# CLI:  target/release/smtp-test-tool
# GUI:  target/release/smtp-test-tool-gui   (built when `gui` feature is on, default)

Linux build dependencies (for the GUI)

sudo apt install -y \
  libxkbcommon-dev libwayland-dev \
  libxcb-render0-dev libxcb-shape0-dev libxcb-xfixes0-dev \
  libgl1-mesa-dev libegl1-mesa-dev libfontconfig-dev

Project layout

src/
├── lib.rs            re-exports + Outlook defaults
├── config.rs         TOML config with named profiles
├── diagnostics.rs    server-response -> human hint translators
├── tls.rs            rustls ClientConfig builder
├── smtp.rs           SMTP test (lettre)
├── imap.rs           IMAP test (hand-rolled on rustls)
├── pop3.rs           POP3 test (hand-rolled on rustls)
├── runner.rs         orchestrator (run enabled protocols, summarise)
└── bin/
    ├── cli.rs        clap-based CLI
    └── gui.rs        eframe / egui GUI

See AGENTS.md for the working agreement every contributor (human or AI) must follow.

Contributing

Issues and PRs welcome. Read AGENTS.md and CONTRIBUTING.md first — they encode the non-negotiable bits (WCAG 2.2 AAA, dark+light mode, atomic conventional commits, latest-stable deps verified against the registry).

License

Dual-licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE)
• MIT license (LICENSE-MIT)

at your option.

Unless you explicitly state otherwise, any contribution you submit for inclusion shall be dual-licensed as above, without any additional terms or conditions.