function emailCheck (emailStr) {

   	/* The following variable tells the rest of the function whether or not
   	to verify that the address ends in a two-letter country or well-known
   	TLD.  1 means check it, 0 means don't. */

	var checkTLD=1;

	// The following is the list of known TLDs that an e-mail address must end with. 

	var knownDomsPat=/^(com|net|org|edu|int|mil|gov|arpa|biz|aero|name|coop|info|pro|museum|tv)$/;

   	/* The following pattern is used to check if the entered e-mail address
   	fits the user@domain format.  It also is used to separate the username
   	from the domain. */

	var emailPat=/^(.+)@(.+)$/;

   	/* The following string represents the pattern for matching all special
   	characters.  We don't want to allow special characters in the address. 
   	These characters include ( ) < > @ , ; : \ " . [ ] */

	var specialChars="\\(\\)><@,;:\\\\\\\"\\.\\[\\]";

   	/* The following string represents the range of characters allowed in a 
   	username or domainname.  It really states which chars aren't allowed.*/

	var validChars="\[^\\s" + specialChars + "\]";

   	/* The following pattern applies if the "user" is a quoted string (in
   	which case, there are no rules about which characters are allowed
   	and which aren't; anything goes).  E.g. "jiminy cricket"@disney.com
   	is a legal e-mail address. */

	var quotedUser="(\"[^\"]*\")";

   	/* The following pattern applies for domains that are IP addresses,
   	rather than symbolic names.  E.g. joe@[123.124.233.4] is a legal
   	e-mail address. NOTE: The square brackets are required. */

	var ipDomainPat=/^\[(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})\]$/;

	/* The following string represents an atom (basically a series of non-special characters.) */

	var atom=validChars + '+';

   	/* The following string represents one word in the typical username.
   	For example, in john.doe@somewhere.com, john and doe are words.
   	Basically, a word is either an atom or quoted string. */

	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. */

		alert("The email address seems incorrect (check @ and .'s)");
		return false;
	}
	var user=matchArray[1];
	var domain=matchArray[2];

	// Start by checking that only basic ASCII characters are in the strings (0-127).

	for (i=0; i<user.length; i++) {
		if (user.charCodeAt(i)>127) {
			alert("The email username contains invalid characters.");
			return false;
   		}
	}
	for (i=0; i<domain.length; i++) {
		if (domain.charCodeAt(i)>127) {
			alert("The email domain name contains invalid characters.");
			return false;
   		}
	}

	// See if "user" is valid 

	if (user.match(userPat)==null) {

		// user is not valid

		alert("The email username doesn't seem to be valid.");
		return false;
	}

	/* 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) {
				alert("The email destination IP address is invalid!");
				return false;
   			}
		}
		return true;
	}

	// Domain is symbolic name.  Check if it's valid.
 
	var atomPat=new RegExp("^" + atom + "$");
	var domArr=domain.split(".");
	var len=domArr.length;
	for (i=0;i<len;i++) {
		if (domArr[i].search(atomPat)==-1) {
			alert("The email domain name does not seem to be valid.");
			return false;
   		}
	}

   	/* domain name seems valid, but now make sure that it ends in a
   	known top-level domain (like com, edu, gov) or a two-letter word,
   	representing country (uk, nl), and that there's a hostname preceding 
   	the domain or country. */

	if (checkTLD && domArr[domArr.length-1].length!=2 && 
		domArr[domArr.length-1].search(knownDomsPat)==-1) {
		alert("The email address must end in a well-known domain or two letter " + "country.");
	return false;
	}

	// Make sure there's a host name preceding the domain.

	if (len<2) {
		alert("The email address is missing a hostname!");
		return false;
	}

	// If we've gotten this far, everything's valid!
	return true;
}