A Maildir is a directory based mail storage format that offers several advantages over mailbox files. Mailbox files must be locked, and therefore they do not permit concurrent mail deliveries. The mailbox file must be locked while a new message is appended to it, otherwise multiple messages being delivered at the same time will trample all over each other. Maildirs do not require locking, and multiple concurrent deliveries can be made to the same Maildir. The rest of the line specifies the command to be executed by the shell.
Courier runs the specified command, and provides the contents of the message on standard input.
Courier waits until the external command completes execution before going to the next delivery instruction. Courier examines the exit code of the external command in order to determine whether the delivery failed, or not. If the external command terminates with the exit code of zero, the next delivery instruction is executed. If the command was the last delivery instruction in the file, the message is considered to be successfully delivered. If the external command terminates with the exit code of 99, any additional delivery instructions in the file are NOT executed, but the message is considered to be successfully delivered.