When working from the Windows command line, you can do a quick test to validate your SMTP connectivity using PowerShell:
c:\> Powershell -executionpolicy bypass PS c:\> Send-MailMessage –to <TO> –from <FROM> –subject "testing123" –body "this is a test" –smtpserver <SMTPServer> -port 25
And if the mail server is accessed over TLS/SSL with SMTP authentication enabled:
PS c:\> Send-MailMessage –to <TO> –from <FROM> –subject "testing456" –body "this is a secure test" –smtpserver <SMTPServer> -port 587 -UseSsl -Credential (Get-Credential)
This is easier than going down to telnet, which is typically not installed on a modern Windows host:
CMD> telnet <SMTPServer> 25 EHLO <CLIENTHOST> MAIL FROM: <FROM> RCPT TO: <TO> DATA Subject: testing123 this is a test . QUIT
If you need to test authenticated SMTP from a Linux host, see my article here.