Locking mailbox - is the main possible reason. It is impossible to corrupt text file. What's your imail version? The only thing you can corrupt is uid file (index for mailbox) linked with your pop3 clients local uid file. Some early imail versions touched by this issue.
But as you may suspect it is impossible to lock a user mailbox forever. As a rule you can determine the problem by the server logs.
Regards,Dmitri Elgin,http://imailzip.com