You'll have to go in and manually modify admin.php. Do a search for "valid_email" - this will bring up the valid_email function.
Look for this line:
if(ereg("^[\._a-z0-9'-]+(\.[_a-z0-9'-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.([a-z]){2,4})$", strtolower($xe))){
Change the 2,4 to 2,6:
if(ereg("^[\._a-z0-9'-]+(\.[_a-z0-9'-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.([a-z]){2,6})$", strtolower($xe))){
This change will be in all future releases.
Regards