首先要创建 Form14 每次有新邮件时，一个新的实例。

Firstly you are creating a new instance of Form14 every time you have a new message.

其次显示和的ShowDialog 做两件完全不同的事情：

Secondly Show and ShowDialog do two very different things:

显示中只是显示了形式，而的ShowDialog 的显示形式为一个模态对话框。这意味着用户不能做任何事情，直到他们解散的形式。

Show just displays the form, whereas ShowDialog displays the form as a modal dialog. This means the user can't do anything else until they dismiss the form.

您需要有形式的一个实例，您可以使用可见属性，以确定是否它的显示与否。所以，你将有：

You need to have a single instance of the form and you can use the Visible property to determine whether it's shown or not. So you would have:

private Form14 frm14;

然后在构造函数中：

Then in the constructor:

frm14 = new Form14();

然后在你的code：

Then in your code:

if (!frm14.Visible)
{
    // Add the message
    frm14.Show();
}