So after having run the Exchange 2010 Hybrid Configuration wizard (HCW), the "tenant.mail.onmicrosoft.com" domain (used for routing purposes between on-prem and Exchange Online) is stamped on on-prem mailbox users and from there replicated to the Office 365 users via DirSync.
But you notice you don't see it under domains in the Office 365 portal:
You wonder what went wrong…
Don’t worry. This is by design. You will see the domain when you connect to the tenant with PowerShell and run:
Get-MsolDomain as well as Get-AcceptedDomain
In addition, the domain appears in the FOPE administration center:
But the “tenant.mail.onmicrosoft.com” domain is filtered out of the Office 365 Portal UI.
Nothing broke… 🙂