When you create a new mailbox in Exchange Online, that mailbox comes with specific settings, features, and protocols enabled. As an Office 365 administrator, you have the ability to go back and modify these settings later if—for instance—you don't want users to have their default mailbox size limit set at 100 GB, or if you want a specific retention policy applied to that mailbox.
If you'd like to take the next step with such customizations and have them automatically applied to mailboxes, then this is the blog post for you. The Office 365 roadmap shows that Microsoft is in the process of making enhancements to mailbox plans.