[root@linux ~]# vi /etc/postfix/virtual 

  :
  :
# AUTHOR(S)
#        Wietse Venema
#        IBM T.J. Watson Research
#        P.O. Box 704
#        Yorktown Heights, NY 10598, USA
#
#                                                                     VIRTUAL(5)
# 最終行に追加
virtual.jp anything
@virtual.jp @mountainbigroad.jp

[root@linux ~]# postmap /etc/postfix/virtual 

[root@linux ~]# vi /etc/postfix/main.cf 

  :
  :
# 最終行に追加
virtual_maps = hash:/etc/postfix/virtual

[root@linux ~]# service postfix restart 
Shutting down postfix:                                     [  OK  ]
Starting postfix:                                          [  OK  ]