how can custmize mail body

The answer depends upon many things including your email client.

What I did:

I got a simple HTML editor and created the template for my code including adding the company logo which gave it a more official-looking appearance. In areas where I wanted to have dynamic data I put it in a format such as %username%. It takes a bit of thinking because if you have several areas of independent dynamic data, you have to be able to create independent variables in your code. Next I flip from visual to HTML raw view, copy it and paste it into a text resource in my project.

Finally I grab the resource and I do replaces in it

String.Replace("%username%", user.ToString());

The last part is where you get dependent on the mail client. You have to dig into the API docs to find out how to specify you are sending an HTML body. The rest is a piece of cake. Just format your mail object the same as you always have.