When migrating a mailbox (Exchange or Office 365) using On Demand Migration for Email (ODME) there is an error about the mailbox quota exceeded and the migration stops.
Error Message error Failed to process attachments on item 'Subject of email'.(Mailbox quota exceeded.)
Either the target mailbox or its recoverable items has exceeded its quota limit.
The error 'Mailbox quota exceeded' indicates that one of the quota limits being returned from the script below is being exceeded.
To remove items from Recoverable Items, the following article from Microsoft has steps to perform this.