SMTP Test Tool

Test an SMTP server connection, TLS mode, authentication, and sender DNS without turning Dreamlit into an open relay.

SMTP settings

Provider

• Custom
• Google Workspace
• Microsoft 365
• Amazon SES
• Resend
• Postmark
• SendGrid
• Mailgun
• Brevo
• Mailtrap

SMTP host/Port

Security

• STARTTLS
• TLS
• Plain

Authenticate with username and password: Use an app password or SMTP token. Dreamlit sends it only to the SMTP server for this test.

Username

Password

From email

DKIM selector

Send a controlled test email

Dreamlit will use a temporary Dreamlit-owned recipient only. You cannot enter arbitrary recipients.

Run SMTP test

SMTP report appears here

Run the test to check the connection, TLS mode, authentication, optional controlled send, and sender-domain DNS readiness. Credentials are never stored or shown back. Test sends only go to a Dreamlit-controlled inbox.

How do you test an SMTP server safely?

Test the connection first, then verify the selected security mode, authenticate with the provider credentials, and only then send a controlled test email to a temporary inbox you own. The useful result is not a raw SMTP transcript; it is a short report that says what failed first and what to fix.

For production readiness, SMTP success is not enough. Check the visible From domain for SPF, DKIM, and DMARC so the server path and sender-domain authentication agree before users receive mail.

How to test SMTP

Run the checks in the order real failures happen.

• Copy the provider SMTP host and port
  Use the exact host and port from the provider dashboard. Port 587 with STARTTLS is the safest default for most providers.

• Choose the matching security mode
  Use STARTTLS for port 587, TLS for port 465, and plain SMTP only when testing a trusted internal setup that does not carry credentials.

• Use an SMTP token or app password
  Many providers reject account passwords. Generate a provider-specific SMTP token or app password before testing authentication.

• Check the sender domain
  Enter the From address and optional DKIM selector so Dreamlit can check SPF, DMARC, and DKIM DNS readiness.

• Send only to the controlled inbox
  If you enable test sending, Dreamlit sends to its own temporary recipient. The tool never accepts arbitrary recipient addresses.

Provider notes

Common SMTP provider settings to verify first.

• Google Workspace and Gmail
  Use smtp.gmail.com on port 587 with STARTTLS and an app password or OAuth-backed SMTP credential. Plain account passwords are often blocked.

• Microsoft 365
  Use smtp.office365.com on port 587 with STARTTLS. Confirm SMTP AUTH is enabled for the mailbox before assuming the password is wrong.

• Amazon SES
  Use the regional SMTP endpoint and SES-generated SMTP credentials. SES SMTP passwords are different from AWS access keys.

• Resend, Postmark, SendGrid, Mailgun, Brevo, Mailtrap
  Use the provider token as the SMTP password and follow that provider's required username format, such as apikey for SendGrid.

Safety model

Useful SMTP testing without an open relay.

• No arbitrary relay testing
  The test-send path is locked to a Dreamlit-owned temporary recipient so the tool cannot be used to send mail to third parties.

• Short-lived credential handling
  Credentials are submitted only for the single SMTP test request. The report stores booleans like password provided, never the password value.

• Short timeouts and safe errors
  The Worker uses short network timeouts and returns concise guidance instead of backend stack traces or raw secret-bearing transcripts.

Common questions

SMTP test questions, answered.

What is an SMTP test tool?
An SMTP test tool checks whether an SMTP server can connect, complete TLS or STARTTLS, authenticate, and optionally accept a test email. Dreamlit also checks SPF, DKIM, and DMARC readiness for the sender domain.

Can I send the test email to my own address?
No. This public tool only sends controlled test messages to a Dreamlit-owned temporary inbox. That keeps the tool useful for diagnostics without becoming an arbitrary relay.

Does Dreamlit store my SMTP password?
No. The password is used only during the single Worker request. Reports show whether a password was provided, but never store or return the password itself.

Which port should I use for SMTP?
Use port 587 with STARTTLS unless your provider says otherwise. Use 465 for implicit TLS. This public Worker-backed tool does not test port 25.

Why did connection pass but authentication fail?
The host and port are reachable, but the provider rejected the username/password pair. Check whether SMTP auth is enabled, whether you need an app password, and whether the provider expects a token-specific username.

Why do I still need SPF, DKIM, and DMARC?
SMTP authentication proves the server accepted your credentials. SPF, DKIM, and DMARC help receiving inboxes trust that the visible From domain is authorized to send the message.

Fix it with Dreamlit

Ship email workflows without managing SMTP.

Dreamlit helps teams build branded, production-ready auth and lifecycle email workflows without wiring provider SMTP settings, templates, and delivery diagnostics by hand.

Build email workflows