for this you first set the mail configuration.

it is done in settings > technical > outgoing mail services > create
here you can set the mail configuration.

for example, 
in case of gmail:

use

smtp server = smtp.gmail.com
port = 465

connection securty = SSL/TLS

username = your gmail user name

password = your gmail password

Then Please add this code in the action which changes the state (stage).

body = "Dear,
Type your content" 

    mail_values = {
        'email_to': partner.email,
        'subject': subject,
        'body_html': body,
        'state': 'outgoing',
        'type': 'email',
        }
mail_id = mail.mail.create(mail_values)
mail_id.send()