IMail employs a built-in locking system for files in the Spool directory to eliminate concurrency problems. Locks are created by modifying the first character of a file name and creating a special file in the same directory as the locked file.
Files in the Spool directory are only locked while critical reads or writes are being performed on the file. Old locks are removed if they are more than one hour old. This means a user may be locked out of accessing a file or a service for up to one hour as a result of a system crash during a critical time period.
It is possible to manually remove a locked file if you are positive that no process is actually accessing that file. One reason for the long time period is to allow for the time required to transmit large files over slow links. For example, the time-out should be long enough to transmit a 2+ megabyte file across a 2400 baud dial-up connection with processing delays caused by the remote end.