HI,

step 1:set incoming email server

step 2: set email alias 

add message_new  function in helpdesk.ticket

use this as example..

@api.model
def message_new(self, msg, custom_values=None):
    body = tools.html2plaintext(msg.get('body'))
    bre = re.match(r"(.*)^-- *$", body, re.MULTILINE | re.DOTALL | re.UNICODE)
    desc = bre.group(1) if bre else None
    parent_customer = self.env['res.partner'].browse([msg.get('author_id')]).commercial_partner_id
    email_split = tools.email_split(msg.get('email_from'))
    values = dict(custom_values or {},
                  reported_time = msg.get('date'),
                  reported_by = msg.get('author_id'),
                  description = desc or body)
    create_context = dict(self.env.context or {})
    ticket = super(HelpdeskTickets, self.with_context(create_context)).message_new(msg, custom_values=values)
    partner_ids = [x for x in ticket._find_partner_from_emails(tools.email_split(msg.get('cc') or '')) if x]
    ticket_contacts = [i for i in partner_ids if i in parent_customer.child_ids.ids]
    ticket.write({'partner_id': parent_customer.id,
                  'partner_name':msg.get('email_from').split('<')[0],
                  'partner_email':email_split[0] if email_split else parent_customer.email,
                  'contact_ids':[(6,0, ticket_contacts)]})
    if partner_ids:
        ticket.message_subscribe(partner_ids)
    return ticket