To forward user’s email,just put an .forward file with the email addr in the user’s home diretory.If this not work for your server,Make sure the .forward file is belong to the user you want to forward email.for example,if you want to forward all email for user testuser, change current working directory to /home/testuser,touch a .forward file.
# cd /home/testuser
# touch .forward
# echo ‘email@example.com’ >> .forward
# chown testuser.testuser .forward
# chmod 664 .forward
if this not work,you may check the log file located at /var/log/maillog for more info.
And keep in mind,only in a Postfix/Sendmail system it is sendmail command that utilies this file.For other situation,you may check the related document.