swaks

1Install it

# macOS
brew install swaks

# Debian / Ubuntu
sudo apt install swaks

2Send a test message

swaks --server smtp.mailclutch.com --port 587 -tls \
  --auth LOGIN \
  --auth-user "mc_0000000000000000" \
  --auth-password "pw_0000000000000000" \
  --from staging@example.com \
  --to anyone@example.com \
  --header "Subject: Sandbox check" \
  --body "Hello from swaks."

Every line of the exchange is printed, so a rejection tells you which command it failed on.

Watch it land

Open the sandbox and the message is there — rendered HTML, plain text, raw source, attachments, and the SPF, DKIM and DMARC verdicts. It updates live, so you can leave it open next to your terminal.

Nothing is ever delivered onward, whatever address you send to; claim the inbox with an account and it stays.

What usually goes wrong

• -tls or -tlsc

  -tls is STARTTLS, for ports 25, 587, 2525. -tlsc is implicit TLS, for 465.

• Sending a real file

  --data ./message.eml sends a message you already have, headers included.

Other stacks

The same four values in another dialect. Every guide below is filled in with the sandbox pair at the top of this page.