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".

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".
• Outlook.com / Office 365 defaults baked in — works out of the box for most accounts.
• 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"

Saving the password is opt-in via the GUI tickbox; even then it is stored base64-encoded (obfuscation, not encryption) and you should protect the file with NTFS ACLs or chmod 600.

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.