What does the error Failed to create spool file mean?

Exim has been unable to create a file in its spool area in which to store an incoming message. This is most likely to be either a permissions problem in the file hierarchy, or a problem with the uid under which Exim is running, though it could be something more drastic such as your disk being full. If you are running Exim with an alternate configuration file using a command such as exim -C altconfig..., remember that the use of -C takes away Exim's root privilege. Check that you have defined the spool directory correctly by running

exim -bP spool_directory

and examining the output. Check the mode of this directory. It should look like this, assuming you are running Exim as user exim:

drwxr-x---   6 exim  exim      512 Jul 16 12:29 /var/spool/exim

If there are any subdirectories already in existence, they should have the same permissions, owner, and group. Check also that you haven't got incorrect permissions on superior directories (for example, /var/spool). Check that you have set up the Exim binary to be setuid root. It should look like this:

-rwsr-xr-x   1 root     xxx       502780 Jul 16 14:16 exim

Note that it is not just the owner that must be root, but also the third permission must be s rather than x.