Thanks for the responses. I has a look over the weekend. I created a VM on Digital Ocean and set up SMTP with Brevo. Everything was okay. 
So nothing wrong with Discourse.
I figure Discourse uses a Ruby GEM such as NET::smtp to mail and that by default there is no smtp server installed within the container. The issue we are facing is on an Azure tenant.
To me it feels like the request to the external SMTP service i.e. being blocked somewhere. By some kind of anti-SPAM mechanism sitting on top of the tenant.
I have a few more tests to carry out to narrow this down.
Any info on how Discourse facilitates email sending would be great I am a bit of a newcomer to Ruby on the Rails.