Test if an email address exist without sending mail

First - Find mail exchanger of reddit.com

COMMAND:
nslookup - q=mx reddit.com
RESPONSE:
reddit.com MX preference = 10, mail exchanger = mail.reddit.com
mail.reddit.com internet address = 208.96.53.70

Second - Connect to mail server mail.reddit.com

COMMAND:
telnet mail.reddit.com 25
RESPONSE:
220 mail.reddit.com ESMTP Postfix NO UCE NO UEMA C=US L=CA Unsolicated electronic mail advertisements strictly prohibited, subject to fine under CA law CBPC 17538.45. This electronic mail service provider’s equipment is located in the State of California. See http://www.reddit.com/static/inbound-email-policy.html for more information.

COMMAND:
helo hi
RESPONSE:
250 mail.reddit.com

COMMAND:
mail from: youremail@gmail.com
RESPONSE:
250 2.1.0 Ok

COMMAND:
rcpt to: mailbox.does.not.exist@reddit.com
RESPONSE:
550 5.1.1 : Recipient address rejected: User unknown in local recipient table

COMMAND:
quit
RESPONSE:
221 2.0.0 Bye