Delivery instructions in .courier are executed one at a time. If the execution of a delivery instruction fails for some reason, the message is either returned as undeliverable, or requeued for another delivery attempt. Messages that remain queued for a long period of time are returned as undeliverable. Blank lines in the file are ignored. The line must specify the complete location of the mailbox file, or a Maildir.
Filenames starting with . are relative to the account's home directory.
A mailbox file is a traditional mailbox file that's readable by most mail software. 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.