I stop execution and return to the same page possible keeping the data

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ASP.NET

I am developing something for a company and the user must register using the company's email address. "someName@company.lc"

I am using the default asp.net 2012 Account Registration.

I want when the user put information in the textbox id=username, that if it does not contain @Company.lc to send a message to them telling to use the correct email address
and give them the oppunitity to change it.

At present it shows the message but continue continue executing.

What I have tried:

<WizardSteps>
           <asp:CreateUserWizardStep runat="server" ID="RegisterUserWizardStep">
               <ContentTemplate>


                   <p class="message-info">
                       Passwords are required to be a minimum of <%: Membership.MinRequiredPasswordLength %> characters in length.
                   </p>

                   <p class="validation-summary-errors">
                       <asp:Literal runat="server" ID="ErrorMessage" />
                   </p>

                   <fieldset>
                       <legend>Registration Form</legend>
                       <ol>
                           <li>
                               <asp:Label runat="server" AssociatedControlID="UserName">User name</asp:Label>
                               <asp:TextBox runat="server" ID="UserName" />
                               <asp:RequiredFieldValidator runat="server" ControlToValidate="UserName"
                                   CssClass="field-validation-error" ErrorMessage="The user name field is required." />
                           </li>
                           <li>
                               <asp:Label runat="server" AssociatedControlID="Email">Email address</asp:Label>
                               <asp:TextBox runat="server" ID="Email" TextMode="Email" />
                               <asp:RequiredFieldValidator runat="server" ControlToValidate="Email"
                                   CssClass="field-validation-error" ErrorMessage="The email address field is required." />
                           </li>
                           <li>
                               <asp:Label runat="server" AssociatedControlID="Password">Password</asp:Label>
                               <asp:TextBox runat="server" ID="Password" TextMode="Password" />
                               <asp:RequiredFieldValidator runat="server" ControlToValidate="Password"
                                   CssClass="field-validation-error" ErrorMessage="The password field is required." />
                           </li>
                           <li>
                               <asp:Label runat="server" AssociatedControlID="ConfirmPassword">Confirm password</asp:Label>
                               <asp:TextBox runat="server" ID="ConfirmPassword" TextMode="Password" />
                               <asp:RequiredFieldValidator runat="server" ControlToValidate="ConfirmPassword"
                                    CssClass="field-validation-error" Display="Dynamic" ErrorMessage="The confirm password field is required." />
                               <asp:CompareValidator runat="server" ControlToCompare="Password" ControlToValidate="ConfirmPassword"
                                    CssClass="field-validation-error" Display="Dynamic" ErrorMessage="The password and confirmation password do not match." />
                           </li>
                       </ol>
                       <asp:Button runat="server" CommandName="MoveNext" Text="Register" />
                   </fieldset>
               </ContentTemplate>
               <CustomNavigationTemplate />

protected void RegisterUser_CreatedUser(object sender, EventArgs e)
        {
                               
            FormsAuthentication.SetAuthCookie(RegisterUser.UserName, createPersistentCookie: false);

            MembershipUser newUser = Membership.GetUser(RegisterUser.UserName);
            //Make sure that the user have company email name@company.lc

            if (newUser.Email.Contains("@company.lc") != true)
            {
                MsgBox("Please used your Company email ending @company.lc ");
                            }
            else if (newUser.Email.Contains("@company.lc") == true)
            {
                SendEmail();
                string continueUrl = RegisterUser.ContinueDestinationPageUrl;
                if (!OpenAuth.IsLocalUrl(continueUrl))
                {
                    continueUrl = "~/";
                }
                Response.Redirect(continueUrl);
            }
        }
        public void MsgBox(String msg)
        {
            Page.ClientScript.RegisterStartupScript(Page.GetType(), "Message Box", "<script language='javascript'> alert('" + msg + "')</script>");
        }