function CheckSingleEmailAddress ( 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. */

  emailStr = emailStr.toLowerCase ();

  gszEmailAddressError = "";

  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)$/;

  /* 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. */

    gszEmailAddressError = "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)
    {
      gszEmailAddressError = "Name portion of your email address contains invalid characters.";
      return false;
    }
  }
  for (i=0; i<domain.length; i++)
  {
    if (domain.charCodeAt(i)>127)
    {
      gszEmailAddressError = "The domain name contains invalid characters.";
      return false;
    }
  }

  // See if "user" is valid

  if (user.match(userPat)==null)
  {
    // user is not valid
    gszEmailAddressError = "Name portion of your email address 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)
      {
        gszEmailAddressError = "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)
    {
      gszEmailAddressError = "The 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)
  {
    gszEmailAddressError = "The 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)
  {
    gszEmailAddressError = "This address is missing a hostname!";
    return false;
  }

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