You may need to modify the contact.aspx.cs file in the root of your blog. See
this thread. I would comment out or remove the "mail.Sender" line:
// mail.Sender = mail.ReplyTo;
You can probably leave the mail.ReplyTo line. This Sender line was removed from contact.aspx.cs in a recent build of BE -- v 18.104.22.168 released on May 16th.