Paypal - a payment process to elucidate - paypal

Question

I have just began working on a project processing paypal payment on a web site, which does not work fine in the sense that the email notification is sent if and only if the user goes back to the site. The fact is that I didn't code it and I even don't know what kind of api it uses. I read the doc which is quite obscure to me (It understood there is a token needs to be sent from the site by get request method but I am really not sure). I understand from the code that, in the following, the op=dodons uri is used to mange the response of paypal and to send an email notification to the user. Here is the only thing I can tell you about the page from which I want to fetch up how the payment is processed :
<form action="{$payPalURL}" method="post" class="payPalForm" id="frmpp">
<input type="hidden" name="cmd" value="_donations" id="typedon"/>
<input type="hidden" name="item_name" value="Donation" />
<input type="hidden" name="business" value="{$myPayPalEmail}" />
<input type="hidden" name="notify_url" value="{$url}/index.php?op=dodons&donation=1&tid={$randomID}&payment=1" />
<input type="hidden" name="return" value="{$url}index.php?page=campaign&op=dodons&campaign_id={$campaign->getId()}&don=ok" />
<input type="hidden" name="rm" value="2" />
<input type="hidden" name="no_note" value="1" />
<input type="hidden" name="cbt" value="Continue" />
<input type="hidden" name="no_shipping" value="1" />
<input type="hidden" name="lc" value="US" />
<input type="hidden" name="currency_code" value="EUR" />
<input type="hidden" name="txn_id" value="{$randomID}" />
<input type="hidden" name="tid" value="{$randomID}" />
{if $logged_user != null}
<input type="hidden" name="a_id" value="{$logged_user->getId()}" />
</form>

Answer 1

It looks like you're using Payments Standard in combination with Payment Data Transfer (PDT).
PDT only works if the user makes it back to the site which is never guaranteed, even with Auto-Return enabled.
Instead, use Instant Payment Notification (IPN). This is a simple POST of all transaction data to a listener script on your server, and this happens regardless of whether or not the user makes it back to your site or not. It's completely separate from the checkout flow.