I tried this solution on Ubuntu.

1. Installation of postfix mail server package. 

Open a a terminal and apply following command,

sudo su     [Enter Linux password]

apt-get install postfix

Use TAB, UP and DOWN ARROW buttons to move between various selections.

Select no configuration in the post installation screen.

2. Copy the configuration file.

cp /usr/share/postfix/main.cf.debian /etc/postfix/main.cf

3. Install following packages too

apt-get install libsasl2-2

apt-get install libsasl2-modules

apt-get install ca-certificates

4. Open the following file and add some lines.

gedit /etc/postfix/main.cf

Add the following lines at the bottom of the file.

relayhost = [smtp.gmail.com]:587

smtp_sasl_auth_enable = yes

smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd

smtp_sasl_security_options = noanonymous

smtp_tls_CAfile = /etc/postfix/cacert.pem

smtp_use_tls = yes

5. Create a new file to save the gmail password.

gedit /etc/postfix/sasl_passwd

Add the following line in the opened file.

[smtp.gmail.com]:587    urgmailid@gmail.com:yourpassword

Add your gmail user name and password.

6. Change the permission of following file.

chmod 400 /etc/postfix/sasl_passwd

7.  Translate the /etc/postfix/sasl_passwd to Postfix lookup tables.

postmap /etc/postfix/sasl_passwd

8. Create the /etc/aliases.db

postalias hash:/etc/aliases

9. Create the /etc/postfix/cacert.pem.

cat /etc/ssl/certs/Equifax_Secure_CA.pem >> /etc/postfix/cacert.pem

10.  Restart Postfix.

/etc/init.d/postfix restart

Check the following settings in Koha to start email notices,

Administration > Patron categories > Overdue notice required > Yes

Koha Administration > Patrons > Enhancedmessagingpreferences > Allow

Set Koha >  Tools > Overdue Notice/status triggers

Enable email service in Koha

koha-email-enable library

Visit following link to check commands to enable email for Koha,

http://wiki.koha-community.org/wiki/Commands_provided_by_the_Debian_packages#E-mail

Information courtesy

These instructions blindly borrowed from Eureka blog.