There should be a client level configuration flag that allows employees to use their e-mail address to login to the system instead of only referencing the Identifiers section. If an employee works across multiple client sites using the same e-mail they should be able to select which client site they wish to login to.
An alternative option would be to allow us to insert the e-mail address into the identifiers section of Employee Setup and allow more than the current 25 character limit since many addresses exceed this.