Send emails to everyone of your linux machine

# vi /etc/alias  <---- edit alias file

add the following line at the bottom of the page
allusers:  user1,user2

update the alias database
# newaliases

Using the above concept you can mail to all users of your office with following line in your
/etc/alias file:

When there are unlimited users
allusers:  user1,user2,user3............. user500

But thats not a smart solution. Each time a new email user created and quit , you need to keep the /etc/alias database update.

1.Mail Forwarding with sendmail

To make your task easy, create an alais entry
# vi /etc/alias
allusers:        :include:/etc/mail/allusers

# newaliases
# touch /etc/mail/allusers

Now each time before sending mail to  alluser@yourdomain.com run the following command in your terminal

# awk -F: '$3 > 100 { print $1 }' /etc/passwd > /etc/mail/allusers

If you dont want to remember this long line you can make a binary file with this command and execute the file before sending mail to  allusers@yourcompany.com

# vi /usr/bin/nameofusers
awk -F: '$3 > 100 { print $1 }' /etc/passwd > /etc/mail/allusers

2. Mail Forwarding with sendmail

# chmod 755 /usr/bin/nameofusers
Now each time before sending mail to  alluser@yourdomain.com run the following command in your terminal

#/usr/bin/nameofusers
It'll send email to those users who are currently listed in /etc/passwd file.