During the migration, you may see the following warning in the log file:
23:57:30 WARNING: [4996-33-99-00004404]Unable to migrate contents of folder '($Trash)'
23:57:30 NIFFindView:
23:57:30 Entry not found in index
Unfortunately, this may not be a warning that is safe to ignore and may cause the entire mailbox migration to fail.
Trash view of the source mail database.
RESOLUTION
Delete and re-create the $Trash folder of the source database. This should allow the source mail database migration.