Column Validation Error Message

Hi all,

I have 2 lists. Both have Customer Id's column. In List2, while adding new item i have to do a validation checking that CustomerId entered exists in List1. If not exits throw an error message. I have created an ItemAdding event receiver in List2 and while saving the item in newform.aspx i am checking whether Customer Id exists in List1 and redirecting to error page. But I don't want to use error page because when error page is thrown i am loosing my newform.aspx. It's not going back to my newform.aspx after throwing my error page.

So, How could i do this?. I want to throw error message and and when error page pop up is canceled it should display newform.aspx. Can i throw error message on newform.aspx itself? without error page. Please guide me and suggest me workaround.

Thanks in Advance.

sam