OpenPop.net get actual message text

I can see that you use the fetchAllEmail example from the OpenPop homepage. A similar example showing how to get body text is also on the homepage.

You might also want to look at how emails are actually structured. A email introduction exists for just this purpose.

Having that said, I would do something similar to the code below.

private void button7_Click(object sender, EventArgs e)
{
    List<OpenPop.Mime.Message> allaEmail = FetchAllMessages(...);

    StringBuilder builder = new StringBuilder();
    foreach(OpenPop.Mime.Message message in allaEmail)
    {
         OpenPop.Mime.MessagePart plainText = message.FindFirstPlainTextVersion();
         if(plainText != null)
         {
             // We found some plaintext!
             builder.Append(plainText.GetBodyAsText());
         } else
         {
             // Might include a part holding html instead
             OpenPop.Mime.MessagePart html = message.FindFirstHtmlVersion();
             if(html != null)
             {
                 // We found some html!
                 builder.Append(html.GetBodyAsText());
             }
         }
    }
    MessageBox.Show(builder.ToString());
}

I hope this can help you on the way. Notice that there is also online documentation for OpenPop.