Forum Discussion
Office 365 Mailbox Storage Limits
What you are describing isn't possible with Exchange Online and I don't think it would be terrible practical anyway. You could go back to the customer and say Exchange Online isn't designed that way and if they really want (need?) unlimited storage, they have to get used to the idea of using the archive mailbox.
A https://support.office.com/en-gb/article/Set-up-an-archive-and-deletion-policy-for-mailboxes-in-your-Office-365-organization-ec3587e4-7b4a-40fb-8fb8-8aa05aeae2ce could be set up to move old emails after a year or two for example, automatically to the archive mailbox. This would make that 100 GB seem larger, as it would free up space for new emails in the primary mailbox.
It would be possible for Microsoft to offer mailboxes > 100 GB but they have elected to do so for very good reason. Mailboxes are usually under dynamic load because messages flow into and out of them on an ongoing basis. Much of the load generated for a mailbox server comes from this activity. On the other hand, archives mailboxes have a write-once, read seldom pattern, which is why you can afford to have really big (expandable) archives. I'm not saying that Microsoft will never have expandable primary mailboxes, but right now it seems like it would be too costly in terms of the server resources that might be needed.