For creating a Main Server, follow the steps mentioned in this site:
http://www.tecmint.com/setup-postfix-mail-server-in-ubuntu-debian/
Some changes and missing stuff on this page:
We assume you are creating a user named info
1) You need to create these folders:
sudo mkdir /home/info
sudo mkdir /home/info/mail
sudo mkdir /home/info/mail/.imap
sudo chown -R info:info /home/info
3) In my case (my server is example.com), I have the HTML folder here:
/var/www/example.com/public_html
So, create the info user here and not where it says:
/var/www/html/info
4) From terminal, you need to add this user to mail:
adduser info mail
5) Give run permission to Postfix by running:
sudo chmod -R +w /var/mail/
6) you will need to edit a configuration file to allow receive emails
sudo nano /etc/postfix/main.cf
Where they put: example change them for example.com
This mistake is done by the wizard.
Restart Postfix using:
/etc/init.d/postfix start
Logs for Postfix:
/var/log/mail.err
NOTE: use sendmailconfig if you changed something on /etc/hosts
NOTE: any problem you have, check mail.err because port 25 or the SMTP could be used.
That should do ;-)
