Use new crm-region instead of overriding smtp settings page
From CiviCRM 4.7.31 there is a new setting "Allow Mail to be sent from logged in contact's email address" on the Outbound mail settings. A new crm-region has been created to allow overriding the default mailer settings: https://github.com/civicrm/civicrm-core/blob/master/templates/CRM/Admin/Form/Setting/Smtp.tpl#L36