Port 25 (TCP) — SMTP Mail Relay
Simple Mail Transfer Protocol (SMTP) on port 25 handles server-to-server (MTA-to-MTA) email relay across the public internet using opportunistic STARTTLS encryption.
| Port Number | 25 |
|---|---|
| Transport Protocol | TCP |
| Primary Service | SMTP Mail Relay |
| Standard / Specification | RFC 5321 |
| IANA Port Range | Well-Known System Port (0–1023) |
| Internet Exposure Risk | Frequently abused for spam; most cloud VPCs throttle or block outbound TCP/25 by default. |
Firewall Architecture & Hardening Best Practices
Only expose inbound TCP/25 on dedicated MX mail gateways protected by SPF, DKIM, and DMARC validation. Client applications should submit mail on port 587 or 465 instead.
Operational Troubleshooting Note
Verify MX reachability and STARTTLS negotiation with openssl s_client -starttls smtp -connect mail.example.com:25.
Quick CLI Connectivity & Firewall Reference
# Test TCP/25 reachability (Linux / macOS / PowerShell) nc -zv target.example.com 25 Test-NetConnection -ComputerName target.example.com -Port 25 # Allow restricted CIDR on Linux UFW firewall ufw allow from 10.0.0.0/8 to any port 25 proto tcp
Planning firewall allowlists or VPN overlays? Use our IPv4 CIDR Calculator, MTU & TCP MSS Calculator, or WireGuard AllowedIPs Generator.
Browse Common Network & Cloud Ports
Port 22 (SSH) Port 25 (SMTP Mail Relay) Port 53 (Domain Name System) Port 67 (DHCPv4 Server) Port 80 (HTTP Web Traffic) Port 123 (Network Time Protocol) Port 161 (SNMP Polling) Port 179 (Border Gateway Protocol) Port 389 (LDAP Directory) Port 443 (HTTPS & HTTP) Port 445 (SMB Direct File Sharing) Port 465 (SMTPS Implicit TLS Submission) Port 500 (IPsec IKEv2 Key Exchange) Port 514 (Syslog Telemetry) Port 587 (SMTP Mail Submission) Port 636 (LDAPS) Port 853 (DNS over TLS) Port 993 (IMAPS Encrypted Mailbox) Port 1194 (OpenVPN Tunnel) Port 1433 (Microsoft SQL Server) Port 1812 (RADIUS Authentication) Port 2049 (Network File System) Port 2379 (etcd Client API) Port 3306 (MySQL & MariaDB) Port 3389 (Remote Desktop Protocol) Port 4500 (IPsec NAT Traversal) Port 5432 (PostgreSQL Database) Port 6379 (Redis) Port 6443 (Kubernetes API Server) Port 51820 (WireGuard VPN)