Both root and debian8 are users in my local machine.
I want to send mail between them with mail command.

debian8@pengsir:~$ echo "hello" | mail -v -s test root@pengsir.hfi
LOG: MAIN
<= debian8@pengsir.hfi U=debian8 P=local S=336
debian8@pengsir:~$ delivering 1ZRKBw-00023L-JP
R: system_aliases for root@pengsir.hfi
R: system_aliases for debian8@pengsir.hfi
R: userforward for debian8@pengsir.hfi
R: procmail for debian8@pengsir.hfi
R: maildrop for debian8@pengsir.hfi
R: lowuid_aliases for debian8@pengsir.hfi (UID 1000)
R: local_user for debian8@pengsir.hfi
T: appendfile for debian8@pengsir.hfi
LOG: MAIN
=> debian8 <root@pengsir.hfi> R=local_user T=mail_spool
LOG: MAIN
Completed
^C
debian8@pengsir:~$ su root
Password:
root@pengsir:/home/debian8# mail
No mail for root

Why can't send mail when logining in as debian8 user to root user?