What is port 25 used for?

Port 25 is the standard TCP port used for SMTP (Mail Server Relay). It is commonly used by Postfix, Exim, Sendmail, Microsoft Exchange. Original standard protocol port for Simple Mail Transfer Protocol server-to-server email routing.

How do I check if port 25 is open?

Enter your public IP or hostname into the scanner tool and click Check. A sub-second raw TCP socket handshake will be initiated from our edge server to port 25 on your target. If the 3-way handshake finishes with SYN-ACK, the port is OPEN. If an RST packet is returned, the host is reachable but the port is CLOSED. If packets time out without response, the port is FILTERED by an ISP or firewall.

Why does port 25 show as closed or filtered?

Residential internet service providers (ISPs) often block inbound connections on port 25 by default to prevent spam, botnets, or unauthorized server hosting. If the scanner reports FILTERED, verify your router port forwarding configuration and confirm with your ISP whether inbound port 25 is restricted.

PORT 25 TCP Mail High Risk

Port 25 (SMTP (Mail Server Relay)) Open Check

Q: Is port 25 safe to expose to the public internet?

Exposing port 25 carries elevated security risks (High risk). Almost universally blocked by residential ISPs and cloud VPS providers to combat botnet spam. Use port 587 or transactional email APIs instead. For remote services, consider placing them behind a secure VPN (such as WireGuard or Tailscale) or restricting inbound access with strict IP firewall rules.

Original standard protocol port for Simple Mail Transfer Protocol server-to-server email routing.

Live Socket Reachability Probe

Pre-configured for Port 25

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Port Specifications & Network Behavior

Standard Port Number 25

Primary Service SMTP (Mail Server Relay)

Transport Protocol TCP

Traffic Classification Mail

Residential ISP Status Commonly filtered/blocked by residential ISPs

Security Exposure High Exposure discouraged without VPN or firewall protection

Security Advisory: SMTP (Mail Server Relay) (Port 25)

Almost universally blocked by residential ISPs and cloud VPS providers to combat botnet spam. Use port 587 or transactional email APIs instead.

Recommendation: Keep port 25 closed to WAN traffic. Connect remotely using WireGuard, Tailscale, or an SSH tunnel.

Port Forwarding & Troubleshooting Checklist

If port 25 returns CLOSED or FILTERED, work through this verified diagnostic checklist:

1 Inspect ISP policies: residential consumer connections block inbound and outbound TCP 25.
2 Verify mail transport agent (Postfix/Exim) is active via 'systemctl status postfix'.
3 For email client submission, configure port 587 with STARTTLS instead.

Verify Host Is Listening Locally

Windows (PowerShell) netstat -ano | findstr :25

Linux / macOS (Bash) ss -tulpn | grep :25