Look for this code in register.asp, about line 110 or so:

If strAutoLogon <> 1 then
		if Request.Form("Email") = "" then 
			Err_Msg = Err_Msg & "<li>You Must give an email address</li>"
		end if
		if EmailField(Request.Form("Email")) = 0 then 
			Err_Msg = Err_Msg & "<li>You Must enter a valid email address</li>"
		end if
		   <-------- ### INSERT CODE HERE ###
	end if




Somewhere around line 118 of register.asp, right between the 2 "end if" statements:

------------------------------------------------------------------
		dim StrResult
		StrResult = split(lcase(request.form("Email")),"@")

		if StrResult(1) = "hotmail.com" then
			Err_Msg = Err_Msg & "<li>To prevent spamming, you cannot use a Hotmail address.<br>"
			Err_Msg = Err_Msg & "Contact the <a href=""mailto:" & strSender & """>forum administrator</a> to appeal this policy."
		end if

		if StrResult(1) = "yahoo.com" then
			Err_Msg = Err_Msg & "<li>To prevent spamming, you cannot use a Yahoo address.<br>"
			Err_Msg = Err_Msg & "Contact the <a href=""mailto:" & strSender & """>forum administrator</a> to appeal this policy."
		end if
-----------------------------------------------------------------

-Quick N' Dirty Email Verification (screens for hotmail & yahoo)
-by Ken Kogler, ken.kogler@curf.edu
-4/21/2001
-Why? Bored on a Saturday...