Is sending emails between different Office 365 account owned by entirely separate entities secure? I.e. The email never leaves the Microsoft network? 

If the two are in the same data centre (E.g. Ireland) then I'd assume this is the case but is it the same between data centres? 

I've seen this: https://technet.microsoft.com/en-GB/library/dn751021(v=exchg.150).aspx for setting up secure connections to services outside of Office 365 but that doesn't cover internal situations.