function invalidEmail (emailStr) { //returns false on email ok, problem description on problem

	var emailPat=/^(.+)@(.+)$/
	
	var specialChars="\\(\\)<>@,;:\\\\\\\"\\.\\[\\]"
	
	var validChars="\[^\\s" + specialChars + "\]"
	
	var quotedUser="(\"[^\"]*\")"
	
	var ipDomainPat=/^\[(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})\]$/
	
	var atom=validChars + '+'
	
	var word="(" + atom + "|" + quotedUser + ")"
	// The following pattern describes the structure of the user
	var userPat=new RegExp("^" + word + "(\\." + word + ")*$")
	/* The following pattern describes the structure of a normal symbolic
	   domain, as opposed to ipDomainPat, shown above. */
	var domainPat=new RegExp("^" + atom + "(\\." + atom +")*$")
	
	
	/* Finally, let's start trying to figure out if the supplied address is
	   valid. */
	
	/* Begin with the coarse pattern to simply break up user@domain into
	   different pieces that are easy to analyze. */
	var matchArray=emailStr.match(emailPat)
	if (matchArray==null) 
	{
	  /* Too many/few @'s or something; basically, this address doesn't
		 even fit the general mould of a valid e-mail address. */
		return "Email address seems incorrect (check @ and .'s)"
	}
	var user=matchArray[1]
	var domain=matchArray[2]
	
	// See if "user" is valid 
	if (user.match(userPat)==null) 	{
		// user is not valid
		return "The username doesn't seem to be valid.";
	}
	
	/* if the e-mail address is at an IP address (as opposed to a symbolic
	   host name) make sure the IP address is valid. */
	var IPArray=domain.match(ipDomainPat)
	if (IPArray!=null) {
		// this is an IP address
		for (var i=1;i<=4;i++) 	{
			if (IPArray[i]>255) {
				return "Destination IP address is invalid!";
			}
		}
	}
	
	// Domain is symbolic name
	var domainArray=domain.match(domainPat)
	if (domainArray==null) {
		return "The domain name doesn't seem to be valid.";

	}
	
	var atomPat=new RegExp(atom,"g")
	var domArr=domain.match(atomPat)
	var len=domArr.length
	if (domArr[domArr.length-1].length<2 || domArr[domArr.length-1].length>5) 	{
	   // the address must end in a two letter or three letter word.
	   return "The address must end in a three-letter domain, or two letter country.";
	}
	
	// Make sure there's a host name preceding the domain.
	if (len<2) 
	{
	   return "This address is missing a hostname!";
	}
	return false;
}