function emailCheck (emailStr) {
     /* 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("Email address seems incorrect (check @ and .'s)")
          return false;
     }else{
          var user=matchArray[1]
          var domain=matchArray[2]

          // See if "user" is valid
          if (user.match(userPat)==null) {
              // user is not valid
              alert("The username doesn't seem to be valid.");
              return false;
          }else{

               /* 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("Destination IP address is invalid!");
                            return false;
                        }
                   }
               }

               // Domain is symbolic name
               var domainArray=domain.match(domainPat)
               if (domainArray==null) {
                    alert("The domain name doesn't seem to be valid.");
                   return false;
               }

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

               /* Now we need to break up the domain to get a count of how many atoms
                  it consists of. */
               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>3) {
                  // the address must end in a two letter or three letter word.
                  alert("The address must end in a three-letter domain, or two letter country.")
                  return false
               }

               // Make sure there's a host name preceding the domain.
               if (len<2) {
                  alert("This address is missing a hostname!");
                  return false;
               }
          }
     }
}