mailgo

mailgo is a Go CLI tool that verifies whether an email address is deliverable by running syntax checks and a live SMTP handshake — giving you a clear verdict before you send.

Motivation

A friend of mine sends cold emails — a lot of them. He kept running into the same problem: a chunk of every list he built was dead weight — bad addresses, catch-all domains that silently swallow messages, and disposable inboxes that go nowhere. He was burning his sender reputation on addresses that were never going to convert.

I looked around for a simple tool he could run from the terminal before importing a list into his outreach tool. Everything I found was either a paid SaaS, a bloated library, or it only checked syntax. So I built mailgo: a single binary that does a real end-to-end check — SMTP handshake, catch-all detection, the works — and tells you exactly why an address is risky or undeliverable.

How It Works

For every email address, mailgo runs a three-stage pipeline:

| Stage | Check | What It Catches | |-------|-------|-----------------| | 1 | Syntax validation | Malformed addresses | | 2 | SMTP handshake | Mailboxes that don't exist | | 3 | Catch-all detection | Servers that accept everything (unreliable) |

Every address gets one of four verdicts: deliverable · undeliverable · risky · unknown

Quick Start

macOS — Homebrew

brew tap Hrid-a/mailgo
brew install mailgo

Windows — Scoop

scoop bucket add mailgo https://github.com/Hrid-a/scoop-mailgo.git
scoop install mailgo

Linux — install script

curl -fsSL https://raw.githubusercontent.com/Hrid-a/mailgo/master/install.sh | sh

npm

npm install -g @mailgo-cli/mailgo

Go install

go install github.com/Hrid-a/mailgo@latest

Pre-built binary

Download the archive for your platform from the Releases page, extract it, and place the binary on your PATH.

Verify it works

mailgo verify [email protected]

Usage

Flags

| Flag | Description | |------|-------------| | --json | Output results as JSON instead of plain text | | --output, -o | Write results to a file (e.g. results.json) |

Single address

$ mailgo verify [email protected]

Email       : [email protected]
Domain      : company.com
Valid       : true
Status      : deliverable
Host Exists : true
Catch-All   : false
Deliverable : true
Full Inbox  : false
Disabled    : false

JSON output

$ mailgo verify [email protected] --json

{
  "email": "[email protected]",
  "domain": "company.com",
  "valid": true,
  "status": "deliverable",
  "smtp": {
    "host_exists": true,
    "catch_all": false,
    "deliverable": true,
    "full_inbox": false,
    "disabled": false
  }
}

Bulk verification

Put one email per line in a .txt file, then:

$ mailgo verify --file emails.txt --output results.json

Results are written to results.json, one JSON object per address.

Verdicts

| Verdict | Meaning | |---------|---------| | deliverable | Mailbox exists and should receive mail | | undeliverable | Bad syntax or SMTP rejected the address | | risky | SMTP connected but RCPT TO was rejected — may be anti-harvesting policy, not a dead address | | unknown | Catch-all domain or SMTP unreachable — can't confirm either way |

Contributing

Clone the repo

git clone https://github.com/Hrid-a/mailgo.git
cd mailgo

Build the binary

go build -o mailgo .

Run the tests

go test ./...

Submit a pull request

Fork the repository and open a pull request to the main branch. If you hit a mail server that produces a false positive or unexpected result, opening an issue with the domain (redacted if needed) is also very helpful.

License

MIT