Microsoft 365 Email Online Archiving: What You Need to Know

Microsoft 365 mailboxes are generally limited to 50 GB. Typically, users who want to retain more than 50 GB of email have needed to export .olm (or .pst) files for archiving, and then delete messages to stay under the limit.

Microsoft 365 offers an Online Archive feature. When enabled for an account, this provides an additional 100 GB of cloud storage in an Online Archive folder. 

• Email older than 2 years is automatically moved into the Online Archive folder.
• Folder organization structure is preserved in the Online Archive.
• Messages stored in the Online Archive can only be accessed using Outlook for desktop or Outlook on the web (rather than the Mail apps on Mac or iPhone, for example).
• It can take 24 hours or more for the Online Archive to begin functioning.

With Microsoft 365 Business Premium, the online archive allows unlimited storage. When the archive grows beyond about 100 GB, the archive becomes an 'auto-expanding' archive, with the following additional behaviors:

• Subfolders may be automatically created as <folder name>_yyyy (Created on mmm dd, yyyy h_mm), where yyyy is the year the messages in the folder were received.
• To search for mail in auto-expanded archive folders, you must search each archived folder or subfolder separately.
• You can't use the Recover Deleted Items feature to recover an item that was deleted from auto-expanded storage.

References

https://support.microsoft.com/en-us/office/manage-email-storage-with-online-archive-mailboxes-1cae7d17-7813-4fe8-8ca2-9a5494e9a721

https://docs.microsoft.com/en-us/microsoft-365/compliance/enable-archive-mailboxes?view=o365-worldwide

https://docs.microsoft.com/en-us/office365/servicedescriptions/exchange-online-archiving-service-description/exchange-online-archiving-service-description