"Migration stopped because the target mailbox size exceeds the allowable limit."
WORKAROUND 1:
Run the following PowerShell to get statistic details for one of the affected mailboxes:
The output this script provided will help determine where the mailbox is full (TotalItemSize) and the prohibited send/receive quota limits (DatabaseProhibitSendReceiveQuota).
If possible, increase the mailbox size limits or archive data to free up some space. Contact Microsoft Support for any size limit changes that can not be performed via PowerShell.
WORKAROUND 2:
Delete recoverable items before restarting the migration. The following article describes how to delete an item in recoverable:
https://support.office.com/en-us/article/Delete-items-in-the-Recoverable-Items-folder-of-cloud-based-mailboxes-on-hold-Admin-Help-a85e1c87-a48e-4715-bfa9-d5275cde67b0#step4
Please be aware that to prevent the excessive growth of the Recoverable Items folder which can lead to its size quota exceeding and consequent migration stop, we highly recommend turning off In-Place Hold and Litigation Hold for every migrated mailbox for the time of migration.
WORKAROUND 3:
If deleting the recoverable items does not fix this issue. Then increase the target limit by adding an archive mailbox and re-migrate the mailbox. The following article describes how to enable an archive mailbox:
https://docs.microsoft.com/en-us/microsoft-365/compliance/enable-archive-mailboxes?view=o365-worldwide
WORKAROUND 4:
Obtain the user's Office 365 mailbox quota:
WORKAROUND 5:
Depending on the mailbox license type for the user in Office 365 there may be the option to upgrade the license to allow more disk space.
WORKAROUND 6:
Remove the user license and re-assign it back.
