Recently a user tried to send an email from 365 to 365 user  ( external not internet ) and got the following bounce back  

Generating server: SYYP282MB0848.AUSP282.PROD.OUTLOOK.COM

Remote Server returned ‘450 4.5.0 Unable to proxy recipient (6a40c324-f14c-4772-8173-153f28bd5a97,c668bfef-4e9a-4374-9709-f7cc3ab2e31e)’

A look at the headers showed it never left 365! The email was resent and did not bounce back , there must of been an internal 365 issue