A new mailbox management experience is now available in Exchange Online.
When you connect to your Exchange Control Panel (https://outlook.office365.com/ecp/) and access the RecipientsMailboxes section (which is the default section showing up when logging on to the ECP), you will see a blue banner inviting you to try the new mailbox experience.
This new experience is redirecting you to the https://admin.exchange.microsoft.com/?landingpage=mailboxes&form=ecp#/mailboxes URL which looks like the new administration experience of the Office 365 administration portal
As you can notice, you can also manage from this new experience migration batches, mail flow (domains management, connectors…) in addition of user and resource mailboxes (no shared mailboxes for now).
Interestingly this new experience is being advertised only from the mailboxes management section of the current ECP, not from the other available sections.