Nullmailer expects that this directory exists (see
073f4e9c5d/doc/nullmailer-send.8 (L185)).
When it doesn't and an email cannot be sent due to a permanent failure
or has been in the queue longer than queuelifetime (7 days), message
"Can't rename file: No such file or directory" starts appearing in the
log and nullmailer never sends "Could not send message" notification.
This means that the user may never learn that his email was not
delivered.
The current `remotes` option is a string option containing nullmailer remote
definitions. However, those definitions may contain secret credentials and
should therefore not be put world-readable in the nix store.
I added a `remotesFile` option, which allows to specify a path to the remotes
definition file instead. This way, the definitions can be kept outside of the
nix store with more secure file permissions.
