ポート 587 — SMTP Submission (TCP)
What is Port 587?
安全なメール送信の最新標準。
This is a well-known port (0–1023) commonly used by core services like SMTP Submission. · RFC 6409
When do you need Port 587 open?
Open on mail servers that accept authenticated email from clients (MUAs). This is the correct port for email applications and clients sending outbound mail — not port 25, which is for server-to-server delivery.
Common uses and software
- Postfix
- Exim
- Gmail SMTP
- Microsoft 365
- SendGrid
- Mailchimp Transactional
- Mailgun
Security
Port 587 requires STARTTLS and authentication. Ensure your mail server rejects connections that do not upgrade to TLS before sending credentials. Configure rate limiting and monitor for authentication failures, which may indicate credential stuffing. Never allow relay from unauthenticated clients.
Is Port 587 safe?
An open port increases your attack surface. Safety depends on how SMTP Submission is configured.
Keep SMTP Submission up to date with security patches. Require strong authentication and restrict access to the minimum necessary IP addresses.
Test connectivity for Port 587 on any host.
How to Open Port 587
To allow incoming connections to Port 587 (SMTP Submission) from outside your network, configure your firewall and add a port forwarding rule on your router pointing to your device's local IP address.
Open Windows Defender Firewall → Advanced Settings → Inbound Rules → New Rule → Port → TCP → Specific local ports: 587 → Allow the connection → Name and save the rule.
netsh advfirewall firewall add rule name="Open Port 587" dir=in action=allow protocol=tcp localport=587sudo ufw allow 587/tcpsudo iptables -A INPUT -p tcp --dport 587 -j ACCEPTLog into your router (192.168.1.1 or 192.168.0.1) → Port Forwarding → Add a new rule: Protocol TCP, External port 587, Internal IP: your device's local IP, Internal port 587 → Save.
How to close Port 587
If you do not need SMTP Submission, block this port in your firewall or router.
To block port 587 on Windows, run the following command in PowerShell or Command Prompt (as Administrator):
netsh advfirewall firewall add rule name="Block Port 587" dir=in action=block protocol=tcp localport=587sudo ufw deny 587/tcpsudo iptables -A INPUT -p tcp --dport 587 -j DROPRemove or disable any port forwarding rule for Port 587.
Frequently Asked Questions
What is Port 587 used for?▾
Port 587 is the default TCP port for SMTP Submission. It is used by servers and applications that need to accept incoming connections for this service. Whether you need it open depends on whether you are actively running a SMTP Submission server — if not, it should remain closed to reduce your attack surface.
Is Port 587 safe to keep open?▾
An open Port 587 increases your attack surface. If you are not actively running SMTP Submission, close this port in your firewall and remove any port forwarding rules. If SMTP Submission must be accessible, restrict access to known IP ranges, keep the service software updated with security patches, and use strong authentication.