[FIX] mail: faster blacklist search

For the special case of is_blacklisted, and due to
the size of the tablers to handle, it's faster to
filter some records rather than searching the whole
res.partner table

closes odoo/odoo#28028

addons/mail/models/mail_channel.py

@@ -340,7 +340,7 @@ class Channel(models.Model):
     @api.multi
     def _notify_email_recipients(self, message, recipient_ids):
         # Excluded Blacklisted
-        whitelist = self.env['res.partner'].sudo().search([('id', 'in', recipient_ids), ('is_blacklisted', '=', False)])
+        whitelist = self.env['res.partner'].sudo().search([('id', 'in', recipient_ids)]).filtered(lambda p: not p.is_blacklisted)
         # real mailing list: multiple recipients (hidden by X-Forge-To)
         if self.alias_domain and self.alias_name:
             return {