Hi,

Inside the mail activity model, you have two fields res_model and res_id which can give you information about the original record to which the activity is linked with.

So using this two fields inside your code, you can achieve the same.

leads_to_write = env['crm.lead'].search([('id', '=', record.res_id)])
leads_to_write({'stage_id': 2})

Also, try changing for records in records to for record in records also using the res_model field in the activity you can ensure the activity is linked with leads.

for record in records:
... if record.res_model == 'crm.lead':
..........leads_to_write = env['crm.lead'].search([('id', '=', record.res_id)])
..........leads_to_write.write({'stage_id': 2})

Thanks