If your site is Apache/PHP it's easy (and I'm sure it is just as easy with IIS/ASP.NET, I just don't know the details). When you process the form, you will be accessing a lot of $_GET['xxx'] or $_POST['xxx'] variables. Build these and some literal text into a string, then email it like so:
<?php
$body = $_POST['name'] . ' has offered ' . $_POST['amount'] . ' for ' . $_POST['purpose'];
$to = 'your_email@your_isp.com';
$subject = 'web donations';
$ok = mail($to, $subject, $body);
if (!$ok)
...
?>
You may need to talk to your hosting provider to get the email setup right, but when they do get it right, this will do exactly what you want.
which reminds me, I need to beat up on a hosting company to get email to work on a site I look after. Moved to a new hosting company a month ago, and they still haven't got it right!
Cheers,
Peter
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