It is possible for a reseller to give their customers more email space (or unlimited) than they got assigned. I don't see a problem here to overprovision storage to customers but at least on customer level there should be an error when exceeding resellers space. You could add a generic error message and that customers should contact their reseller for further information. Probably make this behaviour configurable (require reseller to set fixed space or do the check on customer level)
