Use MailBuilder.ReplyTo property while creating new email:
MailBuilder builder = new MailBuilder();
builder.From.Add(new MailBox("alice@mail.com", "Alice"));
builder.ReplyTo.Add(new MailBox("reply@mail.com"));
builder.To.Add(new MailBox("bob@mail.com", "Bob"));
builder.Subject = "Test";
builder.Text = "This is plain text message.";
IMail email = builder.Create();