Messages marked "Completed" and "Queued for Delivery" do not show in recipients inbox

Hello, 

Recently I have been encountering an issue when sending messages to outlook/hotmail. 

Reading through our mail logs, messages are marked "Completed" and "Queued for Delivery" - however they do not show in recipients inbox or spam (or anywhere else). Whitelisting the domain / email messages are sent from does not resolve the issue

Are there any resources on increasing deliverability to outlook/hotmail in this situation? 

Thanks