Mailbox migrations may stop midway or appear partially completed when using ODM.
Typical symptom is migration stopping after copying a small amount of data (for example 10–15 GB) with Error : the target mailbox size exceeds the allowable limit

This happens when the Recoverable Items folder size in the source mailbox is larger than the quota allowed on the target mailbox.

• Source has higher Recoverable Items quota
• Target has lower Recoverable Items quota
• Recoverable Items folder contains large data (deletions, purges, versions, holds)
• Target mailbox cannot accept full data set

How to Check

1. Check source mailbox quotas
   Get-Mailbox <user> | fl ProhibitSendQuota,ProhibitSendReceiveQuota,RecoverableItemsQuota,RecoverableItemsWarningQuota,CalendarLoggingQuota
2. Check target mailbox quotas
   Get-Mailbox <user> | fl ProhibitSendQuota,ProhibitSendReceiveQuota,RecoverableItemsQuota,RecoverableItemsWarningQuota,CalendarLoggingQuota
3. Check Recoverable Items size on source
   Get-MailboxFolderStatistics <user> -FolderScope RecoverableItems

Look for:
Recoverable Items Root size (for example 47.26 GB)

Example Comparison

Source:
RecoverableItemsQuota = 100 GB
Recoverable Items size = 47.26 GB

Target:
RecoverableItemsQuota = 30 GB

Result:
Target quota is too small to handle full recoverable items data

Resolution

Option 1 (Recommended): Match target quota to source
Set-Mailbox <user> -RecoverableItemsQuota 100GB -RecoverableItemsWarningQuota 90GB

Option 2: Reduce source data

• Clean Deletions folder
• Clean Purges folder
• Reduce Recoverable Items size if possible

Validation

After fixing:

• Re-run migration task
• Verify migration continues past previous stopping point
• Confirm full data transfer completes