To help on the first question, I think it would go something like this.
It's been a long time since I've written a form to submit to another form, it was in PHP, but this is my version of how to do it in asp.net, and retain the enter key submission.
You may have to rename the ID's of the textbox and take out the prefix txt_, I forget what the name attribute is for.
I took advantage of 4.0 clientIDMode as set the controls as static to maintain the proper name and id values.
<form id="form1" runat="server">
<div>
<asp:panel id="panel_emailSubmit" runat="server" style="width: 350px; text-align: left;" defaultbutton="bt_Submit" clientidmode="Static" xmlns:asp="#unknown">
<table cellpadding="0" cellspacing="0" border="0" style="width: 350px;">
<tr>
<td style="width: 350px; height: 96px; text-align: left;">
<asp:textbox runat="server" id="txt_recip" clientidmode="Static">email@domain.co.za</asp:textbox><br />
<asp:textbox runat="server" id="txt_subj" clientidmode="Static">Feedback Form</asp:textbox><br />
<asp:textbox runat="server" id="txt_from" clientidmode="Static">email@domain.co.za</asp:textbox><br />
<asp:textbox runat="server" id="txt_body" clientidmode="Static"></asp:textbox><br />
</td>
</tr>
<tr>
<td style="width: 350px; height: 46px; text-align: left;">
<asp:button runat="server" text="Button" id="bt_Submit" postbackurl="http://their_page.com/mailto/mailto.pl" clientidmode="Static" />
</td>
</tr>
<tr>
<td style="width: 350px; height: 0px; text-align: left;">
<input type="hidden" name="followup-page" value="http://my_follo_page.com/mailto/confirm.html" />
</td>
</tr>
</table>
</asp:panel>
</div>
</form>