Hi,

Python – Your Custom Wizard

from odoo import models, fields, api

class  CustomMailWizard (models.TransientModel):
    _inherit = 'mail.compose.message'

    customer_id = fields.Many2one( 'res.partner' , string= 'Customer' )
    recipient_email = fields.Char(string= 'Recipient Email' )

    @api.multi 
    def  get_mail_values ( self, res_ids ):
        mail_values = super (CustomMailWizard, self).get_mail_values(res_ids)

        for res_id in res_ids:
            mail_value = mail_values[res_id]

            # Override recipients 
            if self.recipient_email:
                mail_value.update({
                    'email_to' : self.recipient_email,   # send to this email directly 
                    'partner_ids' : [( 5 , 0 , 0 )],          # clear partner_ids
                })
            elif self.customer_id:
                mail_value.update({
                    'partner_ids' : [( 4 , self.customer_id. id )],   # fallback to partner
                })

            mail_values[res_id] = mail_value

        return mail_values

How to Open the Wizard with Context

In your button action or code that launches the wizard, pass the fields through context:

return {
     'type' : 'ir.actions.act_window' ,
     'view_type' : 'form' ,
     'view_mode' : 'form' ,
     'res_model' : 'mail.compose.message' ,
     'target' : 'new' ,
     'context' : {
         'default_model' : 'your.model' ,
         'default_res_id' : self id ,
         'default_customer_id' : self.customer_id. id ,
         'default_recipient_email' : self.recipient_email,
         'default_composition_mode' : 'comment' ,
         # Optional subject/body
    }
}

i hope it is use full

if self.recipient_email:
           # If recipient_email is set, use it directly
           ctx['default_email_to'] = self.recipient_email
           # Clear partner_ids to avoid sending to both the partner and the custom email
           ctx['default_partner_ids'] = [] 
elif self.customer_id:
            # Otherwise, use the customer's partner record
           ctx['default_partner_ids'] = self.customer_id.ids

U can use like this 
just pass this context 
in this

return {
            'name': _('Send Email to Employees'),
            'type': 'ir.actions.act_window',
            'res_model': 'mail.compose.message',
            'view_mode': 'form',
            'view_id': False,
            'target': 'new',
            'context': ctx,
        }

And dont forget to add other context as well