Validators should be fired only on button click . And it should not be fired while clicking anywhere on entire form.

<div class="panel-body">
               <div class="form-group col-md-12">
                   <table>
                       <tr>
                           <td>
                               <label for="email" class="col-md-2 text-left">Please Enter Email ID:</label></td>
                           <asp:HiddenField ID="hdnCheckValidation" runat="server"/>
                           <td>
                               <asp:TextBox ID="txtEmailId" class="form-control" runat="server" Width="350px" onKeyPress="edValueKeyPress();"></asp:TextBox>
                               <asp:Button id="btnSave" runat="server" Text="Check Email Validity" class="form-control" ValidationGroup="ChangeStatus" OnClick="btnSendMail_Click" CausesValidation="true"/>
                               <asp:RequiredFieldValidator ID="rfvEmailAddress" runat="server" ControlToValidate="txtEmailId"
                                    ErrorMessage=" *" CssClass="bottom-left" ForeColor="Red" ValidationGroup="ChangeStatus" ></asp:RequiredFieldValidator>
                               <asp:RegularExpressionValidator ID="revEmailAddress" ControlToValidate="txtEmailId"
                                   runat="server" ErrorMessage=" Invalid Email Address" ValidationGroup="ChangeStatus"
                                   ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*" CssClass="bottom-left" ForeColor="Red"></asp:RegularExpressionValidator>
                           </td>
                       </tr>
                   </table>
               </div>