Why can't send mail with command mail or mailx?

I have installed postfix and start postfix service .
I can send email from user root to user debian8 by telnet command.

root@pengsir:/home/debian8# telnet localhost 25
Trying ::1...
Connected to localhost.
Escape character is '^]'.
220 pengsir.hfi ESMTP Postfix (Debian/GNU)
ehlo mail
250-pengsir.hfi
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-8BITMIME
250 DSN
mail from:<root>
250 2.1.0 Ok
rcpt to:<debian8>
250 2.1.5 Ok
data
354 End data with <CR><LF>.<CR><LF>
this is a test mail send from root
.
250 2.0.0 Ok: queued as CA53CF82C3
quit
221 2.0.0 Bye
Connection closed by foreign host.

Let’s change user debian8 to receive the email.

root@pengsir:/home/debian8# su debian8
debian8@pengsir:~$ mail
Mail version 8.1.2 01/15/2001.  Type ? for help.
"/var/mail/debian8": 1 messages 1 new 1 unread
 N  1 root@pengsir.hfi   Wed Aug 19 10:10   13/441   
& 1
Message 1:
From root@pengsir.hfi  Wed Aug 19 10:10:56 2015
X-Original-To: debian8
Date: Wed, 19 Aug 2015 10:10:23 +0800 (HKT)
From: root@pengsir.hfi

this is a test mail send from root

& quit

It is strange that i can not send email with mail or mailx command.

mail -s 'test_for_postfix'  debian8@pengsir.hfi
mailx -s 'test_for_postfix'  debian8@pengsir.hfi

It look like that the cursor will be stop in the console forever ,no error info putput when to execute the two commands.