for (int i = 1; i <= messageList.Count; i++)
{
    // the .Item() method expects either a string or a long
    // see http://www.novell.com/documentation/developer/groupwise_sdk/gwsdk_gwobjapi/data/h20s5bdo.html
    long index = (long)i;

    // instantiate a Message object to get access to the different properties like subject, sender etc
    GroupwareTypeLibrary.Message oMessage = (GroupwareTypeLibrary.Message)ml.Item(index);
    
    // get the email address of the sender 
    MessageBox.Show(oMessage.Sender.EMailAddress);
}