function echeck(str) {

		var at="@"
		var dot="."
		var lat=str.indexOf(at)
		var lstr=str.length
		var ldot=str.indexOf(dot)
		if (str.indexOf(at)==-1){
		   return false
		}

		if (str.indexOf(at)==-1 || str.indexOf(at)==0 || str.indexOf(at)==lstr){
		   return false
		}

		if (str.indexOf(dot)==-1 || str.indexOf(dot)==0 || str.indexOf(dot)==lstr){
		    return false
		}

		 if (str.indexOf(at,(lat+1))!=-1){
		    return false
		 }

		 if (str.substring(lat-1,lat)==dot || str.substring(lat+1,lat+2)==dot){
		    return false
		 }

		 if (str.indexOf(dot,(lat+2))==-1){
		    return false
		 }
		
		 if (str.indexOf(" ")!=-1){
		    return false
		 }

 		 return true					
	}

function CheckData() {
	
	if (echeck(document.frmdk.thanhvien_mail.value) == false){
		alert ('Dia chi email cua ban khong dung. Xin vui long nhap lai.');
		document.frmdk.thanhvien_mail.focus();
		return false;
	}
	 
	
	//document.frmdk.submit();
	return true;
}
<!--
//-------------------------------------------------------  
 function check_number(valuekt,ten)
{	
	var i;var t;var t1;	
	for(i=0;i<eval(valuekt).value.length;i++) {
		t = eval(valuekt).value.substr(i,1);
		t1=parseInt(t,10);
		if((t1!=0)&&(t1!=1)&&(t1!=2)&&(t1!=3)&&(t1!=4)&&(t1!=5)&&(t1!=6)&&(t1!=7)&&(t1!=8)&&(t1!=9))
		{
			alert("Not number , input again " + ten + " !");			
		    eval(valuekt).value="";
			eval(valuekt).focus();
			return false;
		}
	}
	
	return true;
}
//---------------------------------------------
function CheckEmail(emailStr) {
var checkTLD=1;
var knownDomsPat=/^(com|net|org|edu|int|mil|gov|arpa|biz|aero|name|coop|info|pro|museum)$/;
var emailPat=/^(.+)@(.+)$/;
var specialChars="\\(\\)><@,;:\\\\\\\"\\.\\[\\]";
var validChars="\[^\\s" + specialChars + "\]";
var quotedUser="(\"[^\"]*\")";
var ipDomainPat=/^\[(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})\]$/;
var atom=validChars + '+';
var word="(" + atom + "|" + quotedUser + ")";
var userPat=new RegExp("^" + word + "(\\." + word + ")*$");
var domainPat=new RegExp("^" + atom + "(\\." + atom +")*$");
var matchArray=emailStr.match(emailPat);

  if (matchArray==null) {
    //alert("Email Address is invalid!");
    return false;
  }

var user=matchArray[1];
var domain=matchArray[2];

  for (i=0; i<user.length; i++) {
    if (user.charCodeAt(i)>127) {
    //alert("Email Address is invalid!");
      return false;
    }
  }
  for (i=0; i<domain.length; i++) {
    if (domain.charCodeAt(i)>127) {
    //alert("Email Address is invalid!");
      return false;
    }
  }

  if (user.match(userPat)==null) {
    //alert("Email Address is invalid!");
    return false;
  }

var IPArray=domain.match(ipDomainPat);

  if (IPArray!=null) {
    for (var i=1;i<=4;i++) {
      if (IPArray[i]>255) {
    //alert("Email Address is invalid!");
        return false;
      }
    }
    return true;
  }

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("Email Address is invalid!");
      return false;
    }
  }

  if (checkTLD && domArr[domArr.length-1].length!=2 &&
     domArr[domArr.length-1].search(knownDomsPat)==-1) {
    //alert("Email Address is invalid!");
    return false;
  }

  if (len<2) {
    //alert("Email Address is invalid!");
    return false;
  }

  return true;
}


//-->