I modify the function a little (the indexes for Substring call)
Public Class Form1
Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
RichTextBox1.Text = "test@{test.fr}"
Dim i As Integer = RichTextBox1.Text.LastIndexOf("@{")
Dim s As String = RichTextBox1.Text.Substring(i+2, RichTextBox1.Text.LastIndexOf("}") - (i+2))
MessageBox.Show(s)
End Sub
End Class
This code returns "test.fr" in the MessageBox. So, the value between "@{" and "}".