.net Outlook.MailItem.Body.Replace不工作

通过下面的代码，我想将单词“Hello”的每个实例都改为“Hello”。但出于某种原因，这种变化并没有发生，甚至有些电子邮件符合限制，有些电子邮件包含“你好”一词。我做错了什么？如果你认为这个问题不好，请告诉我如何改进

static void ReadMail() {
    Microsoft.Office.Interop.Outlook.Application app = null;
    Microsoft.Office.Interop.Outlook._NameSpace ns = null;
    Microsoft.Office.Interop.Outlook.MAPIFolder inboxFolder = null;
    app = new Microsoft.Office.Interop.Outlook.Application();
    ns = app.GetNamespace("MAPI");
    inboxFolder = ns.GetDefaultFolder(Microsoft.Office.Interop.Outlook.OlDefaultFolders.olFolderInbox);
    string items = string.Empty;
    Outlook.Items UnreadItems = inboxFolder.Items.Restrict("[Unread]=true");


    foreach (var item in UnreadItems){
        dynamic mail = (Outlook.MailItem)item;
        mail.Body.Replace("Hello", "Hello_");
        mail.Save();
     }
}

做以下改变

mail.Body=mail.Body.Replace("Hello", "Hello_");

做以下改变

mail.Body=mail.Body.Replace("Hello", "Hello_");