You have first to set the outgoing mail server on your odoo settings and test it is working

You need also to create an email template to use when sending to employees

here is a code I used to send emails to employees when the status of a document is changed. Employees are assinged to users in the systme

    def mailing(self, cr, uid, ids,vals,context=None):
        domain=[('name','=','MY_TEMPLATE_NAME')]
        templates = self.pool.get('email.template').search(cr,uid,domain)
        if not templates:
            return
        template = self.pool.get('email.template').browse(cr,uid,templates[0])
        if template.email_to:
            #template.body_html = 'this is a test for the x'
            pr_id = self.browse(cr,uid,ids,context=context)[0].id
            self.pool.get('email.template').send_mail(cr, uid, template.id, pr_id, True, context=context)
    def write(self, cr, uid, ids, vals, context=None):
        #for test only, comment when test succeeded
        self.test = False
        if self.test:
            self.mailing(cr,uid,ids,vals,context)
            return 
        #for test only, comment when test succeeded

it is triggered in the write method of a view after changing the status of the document

Here is the email_to field and the body_html fields

email_to
${object.next_resp.email}

body_html
Dear ${object.next_resp.name}, </p> <p> The document ${object.name} status has been changed to ${object.state}, pls check and take the appropriate action </p> 

        'next_resp':fields.many2one('res.users','Next responsible',),