Port 587 — SMTP submission / STARTTLS

TCPEmail & Messaging

Port 587 is for message submission, commonly from a mail client to its provider. Distinguish it from server-to-server relay on port 25.

Registry assignments, software defaults and local listeners are different things. A port number alone does not identify or secure a service.

Configuration and troubleshooting

Use the provider-required STARTTLS and authentication settings. STARTTLS upgrades the SMTP session, unlike implicit TLS on 465.

Important limitations

A port number does not enforce encryption. If TLS negotiation fails, fix the configuration instead of falling back to cleartext passwords.

Read-only checks: run on the server host

Choose the commands for your OS. Check TCP and UDP separately. No result is not a lasting availability guarantee; inspect host and container separately.

Linux

sudo ss -ltnp 'sport = :587'

macOS

sudo lsof -nP -iTCP:587 -sTCP:LISTEN

Windows PowerShell

Get-NetTCPConnection -LocalPort 587 -State Listen

Sources and review

Sources checked on: 2026-09-05

This review covers the explanations and sources on this page, not your device or every community software entry below.

Software Using This Port (3 software)

Community software entries: links are references, not individual verification.

Mail Submission

MailProxy

Email Clients

MailClient

SMTP Submission

MailProtocol

Reference link ↗

Usage Suggestions

Before using port 587, please confirm it is not occupied by other services
If you encounter port conflicts, consider using other ports or stopping the occupying service