There is a BUG in Smartermail thread's.
If I have 40 accounts for mailbox migration (created using the webservices) and 1 accounts have the incorrect password in the source the thread count goes from 5 to 4 concurrent. If have 5 accounts with unknown password in the source the anothers accounts become in the queue forever.
The solution it's to restart the SM and requeue the accounts.
Using the latest SM 14.x version.