Why an Exchange Online mailbox fills up even with an Online Archive: MRM vs. Purview, Retention Holds, and Split Quotas

A user reached out reporting they could neither send nor receive email—which they had tested by sending an email to themselves from the same mailbox. Digging into why a mailbox with an active Online Archive still hit its storage ceiling surfaced three mechanics worth keeping top of mind during any storage escalation.

1. Send and Send-and-Receive Have Separate Quotas

Exchange Online enforces stepped storage thresholds rather than a single cutoff (on Exchange Online Plan 2 / E3 / E5):

• IssueWarningQuota (98 GB): The user receives a warning, but mail flow is unaffected.
• ProhibitSendQuota (99 GB): Outbound mail is blocked first. The user cannot send new messages, but can still receive inbound mail until the mailbox grows another gigabyte.
• ProhibitSendReceiveQuota (100 GB): Both outbound and inbound mail are blocked; external senders receive a 5.2.2 mailbox full NDR.

Because ProhibitSendQuota triggers 1 GB before ProhibitSendReceiveQuota, a user between 99 GB and 100 GB can still receive external email—or conversely, if they test “can I receive mail?” by sending a test email to themselves from the same mailbox, the test fails on the outbound send gate before inbound delivery is ever tested.

(Note: RecoverableItemsQuota is tracked separately at 30 GB—or 100 GB under hold—which is why bulk-emptying a bloated Deleted Items folder can fill Recoverable Items and break deletion itself.)

2. Two Retention Engines Coexist: Exchange MRM vs. Microsoft Purview

Microsoft has been transitioning data lifecycle management from legacy Exchange Messaging Records Management (MRM) to Microsoft Purview retention policies, but the migration is not complete:

• Exchange MRM policies (retention tags linked to a mailbox retention policy, evaluated by the Managed Folder Assistant) are still actively in effect and are what drive Move to Archive (MoveToArchive) behavior for Exchange Online mailboxes.
• Purview retention policies (Get-RetentionCompliancePolicy via Connect-IPPSSession) layer on top of Exchange MRM to govern compliance retention and deletion (Retain, Delete, KeepAndDelete).

If you only check Purview in the compliance portal—or only check Exchange MRM in EAC—you are only seeing half of the retention picture. Also, never trust an MRM tag’s display name: always inspect its actual RetentionAction in PowerShell, as admins sometimes edit a tag’s action without renaming it.

3. RetentionHoldEnabled Silently Stops Online Archiving

When a mailbox has RetentionHoldEnabled: $true (or ElcProcessingDisabled: $true), the Managed Folder Assistant suspends MRM processing for that mailbox. Not only are delete tags paused, archive tags (MoveToArchive) do not kick in either. Nothing moves to the In-Place / Online Archive, even when ArchiveStatus is Active and AutoExpandingArchiveEnabled is $true.

You can spot all three conditions in sixty seconds with two PowerShell checks:

# 1. Inspect quotas, holds, and archive configuration
Get-Mailbox <UPN> | Format-List *Quota*, UseDatabaseQuotaDefaults, LitigationHoldEnabled, `
    InPlaceHolds, RetentionHoldEnabled, ElcProcessingDisabled, RetentionPolicy, `
    ArchiveStatus, AutoExpandingArchiveEnabled, RetainDeletedItemsFor

# 2. Compare actual primary, Recoverable Items, and archive consumption
Get-MailboxStatistics <UPN> | Format-List TotalItemSize, TotalDeletedItemSize
Get-MailboxStatistics <UPN> -Archive | Format-List TotalItemSize

If RetentionHoldEnabled is $true with no documented legal or compliance requirement (after verifying RetentionComment, hold dates, and the unified audit log), clearing the hold (Set-Mailbox <UPN> -RetentionHoldEnabled $false) and triggering Start-ManagedFolderAssistant <UPN> -FullCrawl after replication allows MRM to resume moving aged items into the Online Archive.