Hi, crone + php scrip will work for you.
It is possible to do using IMAP access to your email (all three emails).
Please confirm workflow:
1. script check email
2. If new email from PayPal then script will check subject/body and if match then will send new email using your template - to your customer I guess, not to PayPal...
3. stop working till next time.
Is it correct?
What information do you want to extract from the paypal message and what do you want to send to your customer?
Ready to discuss and start working,
Waiting for your reply.
Denis