-bash-3.2$ telnet newman.eecs.umich.edu 25
Connected to newman.eecs.umich.edu.
Escape character is '^]'.
220 newman.eecs.umich.edu ESMTP Sendmail 8.14.3/8.14.2; Fri, 8 Jan 2010 14:13:46 -0500
250 newman.eecs.umich.edu Hello [184.108.40.206], pleased to meet you
250 2.1.0 <email@example.com>... Sender ok
250 2.1.5 <firstname.lastname@example.org>... Recipient ok
354 Enter mail, end with "." on a line by itself
helo, this is a test.
250 2.0.0 o08JDkHp008049 Message accepted for delivery
Here "helo Z" can be replaced to "helo [your machine's DNS name or IP address]" which conforms to the RFC specification. Some server may reject your email if you do not follow this rule.
"mail from:<email@example.com>" can be replaced with "mail from:<ANY EMAIL ADDRESS>". This is a known problem with SMTP protocol. Most servers nowadays have some mechanisms to protect against it, but clearly this server does not.
"rcpt to:<firstname.lastname@example.org>" can be replaced with "rcpt to:<ANYONE@eecs.umich.edu>". It has to be @eecs.umich.edu because newman.eecs.umich.edu is the mail server serving only accounts with @eecs.umich.edu.
Please do not abuse it, e.g. send me spam messages :)
-bash-3.2$ dig www.google.com
;; QUESTION SECTION:
;www.google.com. IN A
;; ANSWER SECTION:
www.google.com. 241754 IN CNAME www.l.google.com.
www.l.google.com. 65 IN A 220.127.116.11
www.l.google.com. 65 IN A 18.104.22.168
www.l.google.com. 65 IN A 22.214.171.124
www.l.google.com. 65 IN A 126.96.36.199
www.l.google.com. 65 IN A 188.8.131.52
www.l.google.com. 65 IN A 184.108.40.206
dig is a built-in command line tool on *nix platform. Check "man dig" for the manual page.
Wireshark Homepage It's a very useful tool that helps learn computer network a lot.