Mail messages that have not yet been delivered are stored in the sendmail program's queue directory. The location of that directory is defined by the QueueDirectory option. That location can be a relative pathname (for testing) or an absolute pathname. If the specified location does not exist, sendmail prints something such as the following:

cannot chdir(/var/spool/mqueue): No such file or directory

If the location exists but is not a directory, sendmail prints something such as the following:

cannot chdir(/var/spool/mqueue): Not a directory

In both cases, sendmail also logs an error message via syslog(8) if the logging level of the LogLevel option (LogLevel) permits. In both cases, sendmail aborts immediately.

The forms of the QueueDirectory option are as follows:

O QueueDirectory=path     configuration file (V8.7 and later) 
-OQueueDirectory=path     command line (V8.7 and later) 
define(`QUEUE_DIR',`path')    mc configuration (V8.7 and later) 
OQpath                    configuration file (deprecated) 
-oQpath                   command line (deprecated) 

The path argument is of type string. If it is missing, the value for path defaults to mqueue. Relative names for the queue are always relative to the directory in which sendmail was invoked. If the entire QueueDirectory option is missing, the value for path defaults to a null string, and sendmail complains with:

QueueDirectory (Q) option must be set

The default in configuring with the mc technique varies depending on your operating system.

The QueueDirectory option is not safe. If specified from the command line, it can cause sendmail to relinquish its special privileges.