I came across this article on Office 365 Mail Flow Troubleshooting. The diagram done by Dean Corcoran from Microsoft that is included in this post is very well done. I have seen many diagrams over the year and this one creates a simple way to follow a complex process.
Whether you are an messaging administrator using this to in the troubleshooting process of message flow or someone who is learning how SMTP and Office 365 mail delivery work in general. This is one of those great reference tools to print out and have hanging around your desk.