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Port 587 (SMTP Submission) — What It Is and How to Use It

Port 587 is designated for SMTP (Simple Mail Transfer Protocol) message submission. It's crucial for sending emails from a client (like an email application) to an email server, ensuring that outgoing mail is properly authenticated and processed before delivery to its final destination.

Last updated: 2026-06-11

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Port Number 587
Protocol SMTP Submission

Common Use Cases

  • Sending emails from an email client (e.g., Outlook, Thunderbird, Apple Mail) to an email server.
  • Applications or scripts sending automated emails (e.g., notifications, reports) via an SMTP server.
  • Relaying emails from one mail server to another, especially when authentication is required for the submission.
  • Providing a dedicated port for authenticated email submission, separating it from traditional unauthenticated SMTP relay on port 25.

Interactive Command Builder

nc -zv example.com 587

Check if Port 587 is Open

nc -vz <hostname_or_ip> 587
Test-NetConnection -ComputerName <hostname_or_ip> -Port 587
⚠️ Security Note: Port 587 typically requires authentication (username and password) and often uses TLS/SSL encryption (STARTTLS) to secure the communication. Failing to enforce strong authentication and encryption can expose credentials and email content to interception.

Related Ports

Port 25 Port 465 Port 110 Port 995 Port 143 Port 993

Frequently Asked Questions

Is port 587 dangerous to leave open?

Leaving port 587 open on a server is generally not dangerous if it's properly configured to require authentication and use TLS/SSL encryption. Without these security measures, it could be exploited for spam relay or credential harvesting.

What service uses port 587?

Port 587 is used by the SMTP (Simple Mail Transfer Protocol) service specifically for email message submission. This is distinct from the traditional SMTP relay on port 25, as port 587 is intended for client-to-server communication with authentication.