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Exchange and SharePoint Take Different Approaches for Access to Archived Material
In my note about the introduction of 100 GB mailbox quotas for Microsoft 365 Business tenants, I speculated whether enabling archive mailboxes for all Business tenants would be a better idea. Enabling larger mailboxes certainly gets headlines, but it’s also an opportunity for users to store more email online, and that’s not always a good thing.
One of the questions I received after the article appeared was whether Microsoft 365 Copilot could access email stored in archive mailboxes. Microsoft documentation doesn’t say anything definitive on the topic and the situation is a little muddled because the Graph mailbox API doesn’t currently support archive mailboxes. Microsoft announced Graph API support for archive mailboxes as part of its Exchange Web Services deprecation project in MC1469565 (9 Sept 2026) and expects general availability in late October 2026.
However, it’s easy to test Copilot access by asking Copilot to retrieve and summarize some email that’s only in an archive mailbox. I was able to do this with messages relating to a 2012 case in the UK High Court where Google sued Microsoft over a patent violation Google believed to exist in ActiveSync (Figure 1). I was an expert witness for Microsoft, who ended up winning the case.
In any case, Copilot was able to retrieve and summarize the messages from my archive mailbox. It even found and used some external email despite a DLP policy running in the tenant to block external messages from Copilot. I expected those messages to be excluded, but they were still discoverable. One explanation is that the policy changes had not yet been fully reflected in Microsoft Search, especially for the archive items. However, I have no definitive evidence to prove the actual cause.
Microsoft Search is Key
The reason Copilot can access archived email lies in how Microsoft Search indexes Exchange data. In general, if Copilot can find items through Microsoft Search, it can use those items in its processing. This principle explains why DLP policies mark messages from external senders, and why Restricted Content Discovery can mark complete SharePoint sites to prevent Copilot from accessing their content.
Microsoft Search processes items stored in archive mailboxes, whether the standard 100 GB archive mailbox or the collection of “shards” which allow an expanding archive mailbox to grow to 1.5 TB. Outlook clients use the search index to find items (Figure 2).
SharePoint Archiving Hides Items from Copilot
SharePoint Online uses a very different approach to archiving. Instead of functionality built-into the product, SharePoint Online depends on Microsoft 365 Archive, and a tenant must configure a PAYG billing arrangement before archiving is possible. The billing arrangement covers any charges for storage used for archiving data. However, charges might never be incurred because Microsoft 365 Archive offsets its storage requirements against the pool of SharePoint storage available to the tenant.
In addition to a platform difference, when a SharePoint Online site or individual files are archived, the action removes the ability of Microsoft 365 Copilot to use that data in its responses. The recent addition of the ability to move SharePoint and OneDrive files to Microsoft 365 Archive as part of the lifecycle enforced by a Purview retention policy or label is an excellent way for tenants to move old files into the archive mid-way through their retention period. Apart from hiding the old files from Copilot, this approach can also reduce consumption of expensive “hot” SharePoint storage with cheaper “cold” archive storage.
Purview retention policies and labels don’t support Exchange Online archive mailboxes. The lack of support by Purview is one of the reasons why tenants use “legacy” Exchange Online retention policies and tags to clean out old email from primary mailboxes.
Product History the Reason for Differences
Product history lies at the base of the different approaches taken to archiving by Exchange Online and SharePoint Online. Microsoft introduced Exchange Online message records management (MRM) and archive mailboxes in Exchange 2010, so these features have been around for a long time. We also have Outlook’s Archive folder, but the least said about that confusing feature, the better.
SharePoint Online didn’t inherit any archiving capabilities from its on-premises predecessor and had to wait for the arrival of Microsoft 365 Archive in 2023 to have such a capability. Time had moved on by that point and new demands had emerged, notably a need to reduce consumption of “hot” SharePoint storage. I don’t think that Microsoft will create a unified Microsoft 365 Archive that can accommodate all workloads any time soon, but I’ve been known to be wrong!
In any case, Microsoft 365 Copilot can access email stored in Exchange Online archive mailboxes, and that’s all that matters.
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