Port 587: SMTP submission — Complete Guide

Port 587 is reserved by IANA for the SMTP submission service. SMTP submission (explicit STARTTLS). RFC standard for mail clients.

To find out if port 587 is open on your host (or a third-party's — for diagnostics), use our port checker. Enter a domain and a port number — results arrive in 1-2 seconds.

Check port 587 →

An open port is a potential attack vector. General rules:

• Close everything you don't use (firewall default-deny)
• For admin ports (22 SSH, 3389 RDP, 3306 MySQL) — IP whitelist or VPN
• Base cases: 80+443 open for web; 22 — from work/VPN only; DB ports — loopback only
• Use fail2ban for SSH/admin panels

The Simple Mail Transfer Protocol (SMTP) is the backbone of email transmission on the Internet. Port 587 is designated for SMTP submission, which allows email clients to send messages to a mail server. This port is crucial for the secure transmission of emails, as it supports STARTTLS, an extension that enables the use of Transport Layer Security (TLS) to encrypt the connection between the client and the server.

According to the Internet Engineering Task Force (IETF) RFC 6409, which specifies the best practices for email submission, port 587 should be utilized by mail clients for submitting outgoing emails. This is in contrast to port 25, which is primarily used for server-to-server email transmission and is often blocked by ISPs to prevent spam.

Using port 587 ensures that user credentials are securely transmitted to the mail server, protecting against eavesdropping and man-in-the-middle attacks. When a client connects to an SMTP server over port 587, it typically initiates a STARTTLS command, upgrading the connection to a secure one. This is why it is essential for email clients to support this feature to ensure secure email communication.

Checking the accessibility of port 587 is critical for ensuring that your email client can communicate effectively with the mail server. There are several methods to verify if port 587 is open on your network and accessible from your client machine.

• Using Telnet: Open your command line interface and type the following command:

If the connection is successful, you will see a response from the mail server. If it fails, you may receive an error message indicating that the connection is refused or timed out.

• Using Nmap: Network Mapper (Nmap) is a powerful tool for network discovery and security auditing. To check port 587, run the following command:

This will return the status of port 587, indicating whether it is open, closed, or filtered.

• Using Online Port Checkers: There are various online tools available that can check the status of port 587. Simply enter your mail server's IP address or domain name, and the tool will report whether the port is open or closed.

Regardless of the method you choose, ensuring that port 587 is open and accessible is crucial for the reliable delivery of email messages.

Configuring your email client to use port 587 for SMTP submission is essential for ensuring secure and reliable email transmission. Below are examples of how to configure popular email clients to use port 587.

• Microsoft Outlook:
• Open Outlook and navigate to File > Account Settings > Account Settings.
• Select your email account and click Change.
• In the Advanced Settings, go to the Outgoing Server tab.
• Check the box for My outgoing server (SMTP) requires authentication.
• Under the Advanced tab, set the Outgoing server (SMTP) to 587 and select TLS as the encryption type.
• Mozilla Thunderbird:
• Open Thunderbird and go to Account Settings.
• Click on Outgoing Server (SMTP).
• Select your SMTP server and click Edit.
• Set the Port to 587 and choose STARTTLS as the connection security.
• Apple Mail:
• Open Apple Mail and go to Mail > Preferences.
• Select the Accounts tab and choose your email account.
• Under the Server Settings, set the Outgoing Mail Server (SMTP) to 587 and select Use TLS/SSL.

By configuring your email client to use port 587 with STARTTLS, you ensure that your outgoing emails are sent securely, minimizing the risk of interception and unauthorized access.