Why should I use port 587 instead of port 25 to send mail?

Most consumer ISPs block outbound port 25 to prevent spam. Port 587 is specifically designated for legitimate user submission and is never blocked by ISPs.

Does port 587 require authentication?

Yes. All modern mail servers enforce SMTP AUTH on port 587, requiring valid account credentials before dispatching messages.

How do I test if port 587 is open?

Use our Open Port Checker tool or execute 'openssl s_client -connect mail.example.com:587 -starttls smtp' from your terminal.

Home/Open Port Checker/Port 587

Port 465 Port 993

Well-Known Port Reference

Port 587: SMTP Submission (TCP)

Authenticated email message submission port utilizing STARTTLS encryption for end-user mail clients.

Port Number

587

Transport

TCP

Security Risk

Low

Standard / RFC

RFC 6409

Test Port 587 Reachability Live

Verify whether Port 587 is open, closed, or filtered on your server or public IP address.

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Service Architecture & Function

Port 587 is the standardized port for email submission by Mail User Agents (MUAs) such as Outlook, Apple Mail, and webmail frontends. Unlike port 25 (which handles server-to-server relaying), port 587 requires client authentication (SASL username/password) and mandatory STARTTLS encryption before accepting outbound messages.

Security Profile & Hardening

Ensure that plaintext authentication is strictly disallowed until STARTTLS has been successfully negotiated to prevent credential interception.

Example Firewall Commands:

# UFW (Ubuntu/Debian)

sudo ufw allow 587/tcp

# iptables (Direct Kernel)

iptables -A INPUT -p tcp --dport 587 -j ACCEPT

Frequently Asked Questions

Technical answers and configuration advice for Port 587.

Port 587 is the standard submission port where end users connect their email clients to send outbound emails with mandatory authentication and STARTTLS.