Telnet to Mail

Here is a way to telnet to mail server.

telnet 192.168.111.24 25
Trying 192.168.111.24...
Connected to et24 (192.168.111.24).
Escape character is '^]'.
220 mail.embeddedteam.co m ESMTP Postfix
helo 192.168.111.24
250 mail.embeddedteam.co m
mail from:
250 Ok
rcpt to:
250 Ok
data
354 End data with .

hello world
.
250 Ok: queued as 2D40A7D29

There are many options with this type of checking
You can give ehlo instead of helo and the system will show all authentication available.