telnet: TCP port connectivity diagnostics

Although the Telnet client is disabled by default in modern Windows, it remains the fastest way to verify if a specific TCP port is open and accessible on a remote host.

Enable Telnet on Windows
dism /online /Enable-Feature /FeatureName:TelnetClient
Test SMTP port (25)
telnet smtp.company.com 25

Modern alternative: Test-NetConnection

PowerShell — Verify port without Telnet
Test-NetConnection -ComputerName smtp.company.com -Port 25

Common ports to verify

PortService
80 / 443HTTP / HTTPS
25 / 587SMTP (outgoing mail)
1433SQL Server
3389RDP (remote desktop)
5985WinRM (remote management)