This gives users the option of storing the smtp password outside the world-readable Nix store.

This is part of: #24288.

I tested this by configuring ssmtp to send mail via my mandrill account:

{
  networking.defaultMailServer = {
    directDelivery = true;
    root           = "xxx";
    authUser       = "xxx";
    authPassFile   = "xxx"; # I also tested it with: authPass = "xxx";
    hostName       = "smtp.mandrillapp.com";
    useSTARTTLS    = true;
  };
}

and then sending some mail.

I also tested it by setting authPassFile to a non-existing file or a file with too restrictive permissions. In those cases the correct error message is printed:

sendmail: Could not open the AuthPassFile xxx

Setting authPassFile to an empty file correctly produces the following error message:

sendmail: Error while reading a line from the AuthPassFile xxx, or it is empty