Exchange Online archiving and litigation hold

Exchange Online has three overlapping mechanisms for keeping mail longer than a user might want to keep it: archive mailboxes, Purview retention policies, and holds (litigation, eDiscovery, in-place). Knowing which to reach for matters for compliance and capacity.

Archive mailboxes

A user's primary mailbox is 50 or 100 GB. The archive mailbox is a second mailbox that extends that — same user, accessed in Outlook as a separate folder tree. Once enabled (and on plans that support auto-expanding archive, up to 1.5 TB):

• Older items can be moved automatically by the default Messaging Records Management (MRM) policy.
• Users can drag items into the archive themselves.
• Archive content is searchable in Outlook and in Purview.

Archive isn't about compliance — it's primarily a capacity tool for users with very large mailbox histories.

Purview retention policies

Retention policies in Microsoft Purview keep content for a defined period and (optionally) delete it at the end. Applied to Exchange:

• They preserve content even if a user deletes it — items go to a hidden preservation hold folder.
• They can scope to specific mailboxes (manually or via adaptive scopes).
• They define retain only, delete only, or retain then delete.

Retention policies are the modern way to enforce email retention. They replace older Exchange-specific retention tags for most scenarios.

Holds

Holds preserve content for legal or investigation purposes:

• Litigation hold preserves all content in a mailbox indefinitely (or for a set period). Users can still delete items in the UI — content stays in preservation hold and is recoverable.
• eDiscovery hold is per-case and per-custodian, preserving only content relevant to a specific eDiscovery case.
• In-place hold is the legacy version, being retired in favour of the above.

Holds override retention policies — held content can't be deleted by retention.

How they layer

A mailbox can have a 7-year retention policy, be on litigation hold, and have an enabled archive — all at once. Each system is checked independently before any deletion. Effectively, the strictest preservation wins.

Operational watchpoints

• Preservation hold can grow surprisingly large. Monitor mailbox size in reports.
• Removing a hold doesn't immediately delete preserved content — retention still applies.
• Holds survive offboarding: a held mailbox becomes an inactive mailbox, retained without a licence until the hold is released.

For most tenants: a Purview retention policy at 7 years, plus litigation hold on accounts under investigation, plus archive mailboxes for power users. Document the policy and revisit it annually.