After days of trying all sorts of tutorials I finally found a simple solution (not necessary the best) for a functional ubuntu mail server:

sudo aptitude install postfix

next type

sudo dpkg-reconfigure postfix

and configure like this:

Internet Site
yourdomain.com
john (type your ubuntu user)
yourdomain.com, localhost.localdomain, localhost
No
127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/192.168.0.1/24 (192.198.0.1 replace with your server ip address)
0
+
all

next install mail-stack-delivery

sudo aptitude install mail-stack-delivery

At this point you have a working mail server. Next, I configured SquirrelMail and start sending and receaving mail.

This configuration worked with both Apache and Nginx.