I'm using this function to send email from DNN.
Public Shared Function SendEmail(fromAddress As String, senderAddress As String, toAddress As String, subject As String, body As String, attachments As System.Collections.Generic.List(Of System.Net.Mail.Attachment)) As String
Member of DotNetNuke.Services.Mail.Mail
The function asks for fromAddress and senderAddress is one of these supposed to be used as a replyTo?