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Ports 143 and 993 · TCP

Ports 143 and 993: IMAP (993: IMAP over TLS)

Internet Message Access Protocol, how mail apps keep a mailbox in sync. 993 is the encrypted version.

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 the Windows Mail app connecting out to 993 is checking mail.

Should you block it?

Outgoing is fine for a mail app. Prefer 993 to 143.

Check it on your PC

Run this in PowerShell to see what is using port 143 right now, and which program owns it:

Get-NetTCPConnection -LocalPort 143 -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 143,993" -Direction Inbound -Protocol TCP -LocalPort 143,993 -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.