How to test to see if an smtp mail server is available using telnet

open a telnet session as follows:

telnet mailserver 25

then type

helo

if it responds as follows:

220 EXCH02.corp.mailserver.co.nz Microsoft ESMTP MAIL Service ready at Tue, 8 Oct 2019 17:00:30 +1300
helo
250 EXCH02.corp.mailserver.co.nz Hello [172.16.21.10]
451 4.7.0 Timeout waiting for client input

Connection to host lost.

C:\Windows\system32>

then everything is good.

however, if it responds: 

C:\Windows\system32>telnet mailserver 25
Connecting To mailserver...Could not open connection to the host, on port 25: Connect failed

C:\Windows\system32>helo
'helo' is not recognized as an internal or external command,
operable program or batch file.

there is a problem