The integration works as you would expect, an order is placed and our website and paypal communicate without a problem.
The problem only occurs from paypal transactions that did not originate from our website ie. Paypal invoices. When a non-website transaction is processed through Paypal it still sends an IPN to our website handler. The website does not respond due to it not being related to any known transaction and an error is generated within Paypal, I believe this is the 503 error. After a number of these have happened Paypal have an automatic feature that disables IPN notifications. We then have to manually have the IPN feature turned back on again and we go through the cycle again and again. This has been going on for about 2 years now and only recently did we find the relation to non-website transactions.
We have been in touch with Paypal technical support and they have responded saying there is nothing they can do but there are two ways to resolve the issue as listed below.
1-Modify your IPN handler to be able to process trx_type=send_money or with any transaction type .
2-Easiest method is to remove the IPN URL from your IPN profile and dynamically pass it through the notify_url parameter from your website code itself. This way, the IPN message will only be triggered when a transaction occurs from your website.
It is hard for me to imagine considering the number of people who use Magento and Paypal that this would be a bug in the module. I have searched the forum and not found anything specifically relating to the same issue so I'm bemused.
Does anyone have any advice on whether this is possibly a problem with our installation or how we could go about resolving this issue? If this is an unknown bug it would be great to get some help from Magento.
For reference we are using Magento 184.108.40.206 & the "Website Payments Standard" module.