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TCP
Safe
587 SMTP Submission
Port 587 is the standard authenticated submission port for outbound email clients and apps.
Security Context
Port 587 is commonly expected on managed infrastructure, but it still needs scoped exposure, TLS where applicable, and service-specific hardening.
Nmap Check
Use a version probe first so you confirm whether the expected service is really what is listening on this port.
nmap -sV -p 587 <host>Firewall Commands
UFW
sudo ufw allow 587/tcpiptables
sudo iptables -A INPUT -p tcp --dport 587 -j ACCEPT