This is simple but it is very useful procedure to test email functionality.

First connect to your server using telnet

telnet xx.yy.zz.pp 25
Connected to xx.yy.zz.pp.
Escape character is '^]'.
220 xx.yy.zz.pp ESMTP

Initialize communication Enter following line.

EHLO xyz.com

will return like below

250-xx.yy.zz.pp
250-PIPELINING
250-8BITMIME
250-SIZE 0
250 AUTH LOGIN PLAIN CRAM-MD5