Port 587 (SMTP submission)

Mail submission from email clients and applications to their outgoing mail server, with STARTTLS and SMTP AUTH (RFC 6409).

Well-known (system) port, 0-1023 TCP Email Meant to face the internet

What port 587 is used for

Mail submission from email clients and applications to their outgoing mail server, with STARTTLS and SMTP AUTH (RFC 6409).

Security: should port 587 be open?

Meant to face the internet. Expose it on servers that accept mail from your users. Require authentication and TLS before AUTH, and rate-limit logins: stolen mailbox passwords are used to send spam through port 587.

IANA assignment

Service nameProtoDescriptionAssigneeReference

submission TCP Message Submission - RFC 6409

submission UDP Message Submission - RFC 6409

Registry dates for this port: last modified 2011-11-17.

Check what is listening on port 587

Linux: is anything listening on TCP 587?

sudo ss -tlnp 'sport = :587'

Linux / macOS: which process owns the port

sudo lsof -nP -i :587

Older Linux / BSD (net-tools)

sudo netstat -tlnp | grep ':587 '

Windows (Command Prompt)

netstat -ano | findstr :587

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 587 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine

nc -vz -w 5 HOST 587

From Windows

Test-NetConnection HOST -Port 587

Open port 587 in the firewall

ufw

sudo ufw allow 587/tcp

iptables

sudo iptables -A INPUT -p tcp --dport 587 -j ACCEPT

nftables

sudo nft add rule inet filter input tcp dport 587 accept

Windows Firewall

New-NetFirewallRule -DisplayName 'Allow TCP 587' -Direction Inbound -Protocol TCP -LocalPort 587 -Action Allow

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Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.