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Port 587 · TCP

Port 587: SMTP submission (mail sending)

The standard port for a mail app to send email through your provider, upgraded to encryption with STARTTLS.

Protocol
TCP
On Windows
Not part of Windows; installed software uses it
Risk if open to the internet
Low

Seeing it on your PC

Outlook, Thunderbird or a scanner's scan-to-email connecting out to 587.

Should you block it?

Outgoing is fine for a mail app. An unknown program sending on 587 deserves a look.

Check it on your PC

Run this in PowerShell to see what is using port 587 right now, and which program owns it:

Get-NetTCPConnection -LocalPort 587 -ErrorAction SilentlyContinue | Select-Object LocalAddress,LocalPort,RemoteAddress,RemotePort,State,@{n='Process';e={(Get-Process -Id $_.OwningProcess -ErrorAction SilentlyContinue).ProcessName}}

No output means nothing is using the port at the moment. For every connection at once, paste netstat -ano into the netstat explainer.

Block it in Windows Firewall

In PowerShell run as administrator, this stops other computers from connecting to this port on your PC. It does not stop your own programs connecting out.

New-NetFirewallRule -DisplayName "Block inbound TCP 587" -Direction Inbound -Protocol TCP -LocalPort 587 -Action Block

Undo it with Remove-NetFirewallRule -DisplayName "Block inbound …", using the same name. If you depend on the service on your local network, this rule stops that too.