Networking
Risk: Critical
Port 25 — SMTP (Simple Mail Transfer Protocol)
Port 25 is used by Mail Transfer Agents (MTA) like Postfix and Sendmail for server-to-server email routing.
PORT NUMBER
25
PROTOCOL
TCP
DEFAULT PROCESS
postfix, sendmail, exim
FIREWALL TARGET
25/tcp
Instant Terminal Fixes for Port 25
macOS & Linux Terminal
1. Check who is listening on port 25:
sudo ss -tulpn | grep :252. Kill process occupying port 25 (EADDRINUSE fix):
sudo systemctl stop postfix3. UFW Firewall Allow rule:
sudo ufw deny 25/tcpWindows PowerShell (Admin)
1. Check listening port:
Get-NetTCPConnection -LocalPort 252. Force kill process on port 25:
Stop-Service SMTPSVC -Force3. Docker Port Forwarding Mapping:
docker run -p 25:25 -d namshi/smtpCommon Error Encountered
Connection timed out trying to reach mail server on port 25 (Cloud provider blocked)
Security Advisory & Hardening
Almost all residential ISPs and cloud providers (AWS EC2, Google Cloud, DigitalOcean) block outbound port 25 to stop spam. For sending application emails, use submission ports 587 or 465 with authenticated TLS.
Frequently Asked Questions
Port 25 Questions & Answers
Common questions regarding default services, kill commands, and firewall configurations.
Port 25 is commonly used for SMTP (Simple Mail Transfer Protocol). Port 25 is used by Mail Transfer Agents (MTA) like Postfix and Sendmail for server-to-server email routing.