function checkInput(F)
{
		var errorMessage, inputSender = F.address.value, emailCheck1, emailCheck2;
			
		if (F.sender.value != "" && F.address.value != "" && F.subject.value != "" && F.message.value != "")
		{
			emailCheck1 = inputSender.indexOf('@', 0);
			emailCheck2 = inputSender.indexOf('.', emailCheck1);
		
			if ((emailCheck1 == -1) || (emailCheck2 == -1))
			{
				errorMessage = "Er is geen geldig e-mail adres ingevoerd.";
			}
		}
			
		else
			errorMessage = "\n Niet alle verplichte velden zijn ingevuld.";
							
		if (errorMessage)
				alert(errorMessage);		
		else
			F.submit();	
}