Hello!

I found a solution that I publish here in case it helps somebody else in the same situation.

I had tried to add a Label to the controls collection with no success. The problem was that I was creating a EPiServer.XForms.WebControls.Label which for some reason couldn't be added. Changing to a System.Web.UI.WebControls.Label worked fine and the error message was displayed on the page. The code looks like this:

  public void XForm_BeforeSubmitPostedData(object sender, SaveFormDataEventArgs e)
        {
            XFormControl control = (XFormControl)sender;

            PageBase currentPage = control.Page as PageBase;

            if (currentPage == null)
            {
                return;
            }
            //We set the current page that the form has been posted from
            //This might differ from the actual page that the form property exists on.
            e.FormData.PageGuid = currentPage.CurrentPage.PageGuid;

            //Validation of email addresses
            string emailAddress = e.FormData.GetValue("Email");
            string repeatEmailAddress = e.FormData.GetValue("RepeatEmail");

            if (emailAddress != null && repeatEmailAddress != null)
            {
               if (!emailAddress.Equals(repeatEmailAddress))
                {
                    control.Controls.Add(CreateErrorLabel(CurrentBlock.ErrorMessage,"error"));
                    e.CancelSubmit = true;
                }
            }
        }

 And the CreateErrorLabel method:

private Label CreateErrorLabel(string labelText, string cssClass)
        {
            Label label = new Label();
            label.CssClass = cssClass;
            label.BackColor = Color.White;
            label.Text = labelText;

            return label;
        }