Port 587 – SMTP Submission Protocol Specification

Technical analysis, security advisories, vulnerability mitigation, and firewall syntax for TCP/UDP port 587 (SMTP Submission).

Port Number 587

Transport Protocol TCP

Port Range Type Well-Known (0-1023)

Default Status Restricted / LAN

Service Description & Architectural Context

Mail submission port used by mail client software (MUAs).

IANA / RFC Designation:

Port 587 is assigned to SMTP Submission under the Internet Assigned Numbers Authority (IANA) service name registry. In enterprise infrastructure and homelab setups, services listening on this port must follow the principle of least privilege.

Security Analysis & Attack Surface Assessment

Requires client authentication (SASL) and mandatory STARTTLS.

Hardening Recommendations:

Do not expose port 587 to public WAN interfaces unless authenticated TLS or VPN wrapping is strictly enforced.
Enforce rate-limiting using Fail2ban, CrowdSec, or cloud edge DDoS protection.
Audit active listening sockets locally: ss -tulpn | grep :587 or netstat -ano | findstr :587.

Firewall Rule Snippets for Port 587

UFW (Ubuntu / Debian Linux)

sudo ufw allow 587/tcp

iptables (Standard Linux Kernel)

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

Windows PowerShell (Defender Advanced Firewall)

New-NetFirewallRule -DisplayName "Allow Port 587" -Direction Inbound -LocalPort 587 -Protocol TCP -Action Allow

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