Many people assume that Exchange Online Archiving is the license you would use for backing up the data of an employee who has left the company. However, that's not the case at all.
Instead, Microsoft offers the Exchange Online Archiving license as an add-on option for the on-premises Exchange server to keep old mail on a long-term basis without this counting as the user's mailbox quota. A typical example would be to archive all mail that is more than 2 years old. While this method of preserving historical mail may be appropriate in hybrid environments, it is not a suitable option for Office 365 customers who are entirely in the cloud. In such cases, mailbox retention (in-place hold), third-party backup solutions, ...
