像这样的东西：

var mails = textBoxcontent.split(';'); // you can also split by blanks. You may also consider the use of trim(str) -> see example below

for(var i = 0,len = mails.length; i < len; i++){
    // check mails[i]
    if(false)
        alert();
}

// or

var Failed = '';
for(var i = 0,len = mails.length; i < len; i++){
    // check mails[i]
    if(false)
        Failed += mails[i] + ' ';
}

自定义修剪实现(如果你使用它,jQuery有一个自己的修剪实现)

function trim (str) {
    return str.replace (/^\s+/,'').replace (/\s+$/,'');
}