PORT 25 / TCPEmail & MessagingExposure Risk: High

Port 25: SMTP — Simple Mail Transfer Protocol (Server-to-Server Relay)

Standard port for relaying email between mail servers (MTAs). Often blocked by cloud providers (AWS, GCP, DigitalOcean) to prevent spam.

Default Service

SMTP

Process: postfix / exim

Transport Protocol

TCP

IANA Registered Port 25

Docker Port Flag

-p 25:25

Container to Host Mapping

How to Check & Kill Process on Port 25 (EADDRINUSE Fix)

macOS & Linux (lsof / kill)

# Find process using port 25
sudo lsof -i :25

# Instantly kill process on port 25
lsof -ti:25 | xargs kill -9

Windows (PowerShell & CMD)

# Find PID listening on port 25
netstat -ano | findstr :25

# Force kill by PID
taskkill /PID <PID_NUMBER> /F

Cross-Platform Node.js One-Liner

# Kill port 25 via npx
npx kill-port 25

# Or diagnose via Castov CLI
npx castov fix "EADDRINUSE :::25"

Linux Firewall (UFW) & Connectivity Test

# Test if port 25 is reachable
nc -zv localhost 25

# Allow or deny port 25 in UFW
sudo ufw allow 25/tcp

Frequently Asked Questions about Port 25

What is Port 25 used for?

Port 25 is the standard network port for SMTP (Simple Mail Transfer Protocol (Server-to-Server Relay)) operating over TCP. The typical system daemon or process is postfix / exim.

How do I kill a process running on Port 25 (EADDRINUSE)?

On macOS and Linux, run: lsof -ti:25 | xargs kill -9 (or npx kill-port 25). On Windows PowerShell/CMD, run: netstat -ano | findstr :25 and then taskkill /PID <PID> /F.

Is it safe to expose Port 25 to the internet?

Port 25 has a High security risk profile. Standard port for relaying email between mail servers (MTAs). Often blocked by cloud providers (AWS, GCP, DigitalOcean) to prevent spam.