I have a form which needs an error message attached to a field. Since the the form is filled out in Japanese (two-byte characters), some shoppers forget to change their input mode into single-byte characters when inputting their email address. What is wrong with this is that the automatic order confirmation emails never make it to the shopper.

Can anyone tell me how to code it so that an error message is displayed when someone tries to enter two-byte characters in a particular field?

The page the form is on is in .asp but the error message could be in JS.