You can't response.write literal values in asp.net. Back in the old days, folks would use response.write to create HTML in classic ASP.
The proper way to do it is to use onClientClick, which will fire your Javascript first, before firing a handler such as a postback event, that way you can cancel the postback if the Javascript doesn't like the value.
button.onClientClick = "alert('This EmailID Already Exist. Please Enter valid EmailID'; return false;)"
That will fire the message box in Javascript, but best practice is to actually write a more complex Javascript function, that will validate the data, and then fire the messagebox, and return true to fire the postback event, or false to cancel it.
One of the proper uses for response.write is to respond to a HTTP page request in code, and reply response.write( "Error 500" ).