Need help with email validation

I have an asp form with the following code:

<p><form action="aspform.asp"  formname="myform" method="post" >

              <p><span class="style11">email</span>:

                <input name="firstname" type="hidden" value='<%=session("firstname")%>' />

                <input name="lastname" type="hidden" value='<%=session("lastname")%>' />

                <input name="acct" type="hidden" value='<%=session("acct")%>' />

                <input name="password" type="hidden" value='<%=session("password")%>' />

                <input name="coupon" type="hidden" value='<%=session("coupon")%>' />

                <input name="textfield" type="required" class="style1" id="textfield" size="50" />

                <span class="style6 style2">

                              <input type="submit" name="Submit" id="Submit" value="Submit" />

The last field "textfield" is for entry of an email address.  How can I validate that without creating a separate form?  I need to keep all of the fields in tact as they pass on to my "aspform.asp" which posts the data to a database.

Any help would be appreciated.