Networking
Risk: Medium
Port 587 — SMTP Submission (STARTTLS)
Port 587 is the modern standard port for email client-to-server message submission (e.g. SendGrid, Mailgun, Amazon SES) protected by mandatory STARTTLS encryption.
PORT NUMBER
587
PROTOCOL
TCP
DEFAULT PROCESS
postfix, exim4
FIREWALL TARGET
587/tcp
Instant Terminal Fixes for Port 587
macOS & Linux Terminal
1. Check who is listening on port 587:
openssl s_client -starttls smtp -connect smtp.gmail.com:5872. Kill process occupying port 587 (EADDRINUSE fix):
sudo systemctl restart postfix3. UFW Firewall Allow rule:
sudo ufw allow 587/tcpWindows PowerShell (Admin)
1. Check listening port:
Test-NetConnection -ComputerName smtp.sendgrid.net -Port 5872. Force kill process on port 587:
Get-NetTCPConnection -LocalPort 5873. Docker Port Forwarding Mapping:
docker run -p 587:587 -d my-mail-serverCommon Error Encountered
535 5.7.8 Error: authentication failed on SMTP port 587
Security Advisory & Hardening
Requires SMTP authentication before relaying any messages to prevent open-relay abuse.
Frequently Asked Questions
Port 587 Questions & Answers
Common questions regarding default services, kill commands, and firewall configurations.
Port 587 is commonly used for SMTP Submission (STARTTLS). Port 587 is the modern standard port for email client-to-server message submission (e.g. SendGrid, Mailgun, Amazon SES) protected by mandatory STARTTLS encryption.