Exchange Online Archiving Explained: MRM Policy vs Archive Mailbox vs Purview Retention

If your Exchange Online mailboxes are growing fast, Outlook search feels slow, or users keep asking “Why is my mailbox full?”—you’re already dealing with archiving, whether you realize it or not. The problem is, Microsoft 365 uses multiple terms that sound similar but mean very different things: MRM policy, Archive Mailbox, and Microsoft Purview Retention. Admins often configure one thinking it will solve everything, while users get confused when older emails “disappear” into the archive or still show up in searches. In this post, we’ll break down archiving in the simplest way possible—what each feature actually does, when to use it, and how to design a setup that keeps mailboxes clean without breaking compliance requirements.

MRM stands for Messaging Records Management.

MRM policies are Exchange policies designed for mailbox “housekeeping”, using:

Retention Tags

Rules like:

• Move to Archive after 365 days
• Delete Deleted Items after 30 days
• Delete Junk after 10 days
• Never delete (personal tag)

These tags are grouped under an MRM Retention Policy and applied to mailboxes.

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MRM Policy = automatic cleanup + automatic archiving rules

Quick Recommendation

• Use Archiving/MRM for mailbox size & performance
• Use Purview retention for compliance retention
• Use Litigation hold/eDiscovery hold for investigations

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What does archiving actually do? (Detailed Explanation)

Archiving in Exchange Online is not a backup, and it’s not about “locking” data for compliance.
Archiving is mainly about mailbox organization and size control.

When archiving is enabled and an archive policy (MRM retention policy) runs, Exchange automatically moves older messages out of the user’s primary mailbox and places them into the user’s Archive mailbox.

So instead of a user’s Inbox growing forever, older content is shifted to a separate mailbox store.

✅ What happens when an archive policy runs?

✅ Rule format

An archive rule typically looks like this:

“Move items to Archive after X days”

Example:

• Move Inbox items older than 730 days (2 years) to the Archive mailbox.

Step-by-step: How it works in real life

1) User receives or sends emails normally

Emails arrive in:

• Inbox
• Sent Items
• Custom folders

Nothing changes for the user initially.

2) Exchange checks the age of items

Exchange evaluates items based on their retention age, which is usually calculated from:

• Date received (for incoming email)
• Date created/sent (for some item types)
• Last modified date (in some cases, depending on item behavior)

3) Once items cross the configured age, they are moved

If the rule says:

✅ “Move items older than 2 years to Archive”

Then the system shifts those emails from the primary mailbox into the Archive mailbox.

So:

Primary mailbox gets lighter
✅ Outlook feels faster
✅ Searching becomes cleaner
✅ Users stop hitting mailbox size limits quickly

What does the user see?

From the user’s perspective:

• Email is not deleted
• Email is still accessible
• But it appears under a new mailbox section called:

📌 In-Place Archive (Online Archive)

So the user can still open and read older emails, but they’re stored in the archive.

This is why archiving is considered “safe” from a user productivity point of view:
✅ mail is not lost, only moved.

Important: Archive mailbox is NOT the same as a folder

Many people think archive means:

“Emails go to an Archive folder inside Inbox.”

No.

An Exchange Online archive is basically:
✅ a separate mailbox store attached to the same user.

So you’ll see something like:

• User Mailbox (Primary)
• Online Archive – User (Archive)

✅ Example 1: Inbox → Move items older than 2 years to Archive

What it means

Any email sitting in Inbox that is older than 2 years will move automatically to the Archive mailbox.

Why this is useful

Inbox is usually the biggest folder and grows forever.

This policy:
✅ keeps Inbox clean
✅ improves performance
✅ avoids mailbox quota complaints
✅ allows user to still access old conversations

✅ Example 2: Deleted Items → Delete after 30 days

This is a different type of retention action.

What it means

Items sitting in Deleted Items longer than 30 days are cleaned up automatically.

It is usually applied because:

• Deleted Items becomes huge
• Users “store junk” there
• It wastes space

So after 30 days, Exchange attempts to remove those items.

⚠️ Important note about “Delete after 30 days”

Deleting via MRM policy means Exchange will remove items from normal mailbox folders.

But whether it is recoverable depends on:

✅ Deleted Items retention settings
✅ Litigation Hold / eDiscovery hold
✅ Purview retention policy

So:

• Archiving moves items
• Deletion policies remove items
• Retention/Hold policies preserve evidence

These are separate concepts.

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Archiving is mainly used for mailbox size management

Admins enable archiving when:

• users have huge mailboxes (20GB, 40GB, 80GB+)
• Outlook performance becomes slow
• search results become messy
• organization wants inbox discipline without deleting mail

Does archiving affect compliance retention?

Not directly.

Archiving is not designed to meet compliance requirements.

It is designed to:

• manage storage
• manage mailbox experience

For compliance retention, you still use:

• Purview retention
• Litigation/eDiscovery holds when needed

✅ Quick summary (super simple)

When an archive policy runs:

• Old emails are MOVED to the Archive mailbox
• Users can still access them
• Primary mailbox stays small and fast

Example:

• Inbox > 2 years → Archive
• Deleted Items > 30 days → Delete

Archiving is NOT legal preservation

Archiving in Exchange Online is often misunderstood as a “safe storage” or “compliance solution,” but it’s not built for that purpose.

Archiving changes location, not protection

When archiving is enabled, Exchange simply moves older items from the primary mailbox to the Archive mailbox to reduce mailbox size and improve performance.

It does not change the security or compliance behavior of the messages.

That means:

• Users can still delete emails from the Archive mailbox
• Emails can still be modified, moved, or cleaned up
• Unless something else is configured, deleted items can still be removed permanently

So archiving is “storage management,” not “evidence preservation.”

A real-world example (why archiving fails for investigations)

Scenario:

A finance user is under investigation. Their mail is already archived (older than 2 years).

Now if the user deletes emails from the Archive mailbox:

✅ the emails disappear for the user
❌ archiving does not prevent deletion
❌ archiving does not guarantee recovery as legal evidence

To legally preserve that data, you still need:

Purview retention (policy-based compliance)
or
Litigation Hold / eDiscovery hold (investigation preservation)

The key point

Archiving helps you manage mailbox size and user experience.
Retention and holds are what protect you during audits, legal cases, and investigations.

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MRM Archiving vs Purview Retention (Difference)

MRM / Archiving (Exchange retention tags + archive mailbox)

MRM is an Exchange-only mailbox management feature used to control mailbox size and structure.

It is mainly used to:

• move older emails from the primary mailbox to the Archive mailbox
• clean up folders (example: delete Deleted Items after 30 days)
• improve Outlook performance and user experience

📌 Goal: Keep mailboxes organized and prevent storage complaints.

Purview Retention (Microsoft 365 Compliance)

Purview retention is a compliance-first governance solution that defines how long data must be retained (or deleted) across Microsoft 365.

It is designed to:

• enforce tenant-wide retention rules (example: keep mail for 7 years)
• support compliance and audit requirements
• work across workloads like Exchange, SharePoint, OneDrive, and Teams (depending on scope)

📌 Goal: Meet legal/regulatory retention requirements and ensure consistent compliance.

Final takeaway (one line)

• MRM Archiving = mailbox performance + cleanup
• Purview Retention = compliance retention + governance

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When should you use Exchange Online archiving?

Exchange Online archiving is useful when your goal is to keep the primary mailbox smaller, faster, and easier to manage, while still allowing users to access older emails.

1) Large mailboxes causing storage pressure

If users regularly hit mailbox limits or receive “mailbox full” warnings, archiving helps by moving older mail out of the primary mailbox into the archive.

2) Outlook performance issues

Very large primary mailboxes can slow down Outlook search, indexing, and sync performance. Archiving improves the day-to-day experience by reducing the amount of mail stored in the primary mailbox.

3) Users keep years of email in Inbox

Some users never delete or organize mail and store everything in Inbox forever. Archiving allows them to keep old messages without letting the main mailbox grow endlessly.

4) Need automatic cleanup and lifecycle management

Archiving works well with folder cleanup rules like removing old Deleted Items or Junk Email, so IT doesn’t have to rely on manual mailbox cleanup.

5) Reduce primary mailbox size without deleting business mail

This is the most common reason to enable archiving: old email remains accessible, but it no longer sits in the primary mailbox.

Recommended Best Practice Setup (Practical)

A clean and practical setup usually includes three layers: enable the archive mailbox, automate movement using MRM, and enforce compliance retention using Purview.

Step 1: Enable the Archive Mailbox

Enable the archive mailbox for users who need long-term email storage (typically heavy email users or departments that retain mail for years). This creates a separate Online Archive mailbox so older emails can be stored without bloating the primary mailbox.

Step 2: Apply an MRM policy (Archiving rule)

Apply an Exchange MRM retention policy that automatically moves older email from the primary mailbox into the archive.

Example rule:

• Move mail older than 1 year to the Archive mailbox

This improves mailbox performance and reduces “mailbox full” issues without deleting business-critical emails.

Step 3: Use Microsoft Purview retention for compliance

Archiving helps with mailbox size and organization, but it is not designed for compliance. Use Purview retention policies to enforce how long data must be retained (for example, 7 years) across the organization.

Archiving keeps mailboxes tidy; Purview retention ensures your organization stays compliant.

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Common misconceptions

“If I enable archive, I’m compliant.”

Not necessarily.
An archive mailbox mainly helps with storage organization and mailbox size management. It moves older mail out of the primary mailbox, but it does not automatically enforce compliance retention requirements on its own.

“MRM policies are not the same as compliance retention.”

Not always.
MRM policies are designed for mailbox lifecycle actions (move to archive, delete after X days, cleanup folders). In many cases, MRM can remove content over time. If your goal is legal or regulatory retention, compliance retention should be managed using Microsoft Purview retention, not only MRM.

Final Summary

Best practice: Use archiving to manage mailbox size and user experience, and use Purview retention to meet compliance and legal retention requirements.

Archive Mailbox is a secondary mailbox used to store older emails without overloading the primary mailbox.

MRM Policy controls mailbox lifecycle actions like moving items to the archive or cleaning up old content automatically.

Microsoft Purview retention provides compliance-based retention across Microsoft 365 workloads such as Exchange, SharePoint, OneDrive, and more.

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FAQ: Exchange Online Archiving (MRM) vs Archive Mailbox vs Purview Retention

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