[Kall]

If you are using paypal with an another application for payments (eg. vbulletin paid member subscriptions), you may have noticed that your subscriptions are failing since October 11, 2006. This happens because apparently paypal changed its API while contacting external scripts for payment transactions.

This might not affect you if :
a) you have a single paypal email account defined
b) if you are using your primary paypal account in your application as your payment email.
c) If your application does not make a "incoming email security check" when paypal API triggers it.

However it can affect you if:
a) You have multiple email accounts (eg. sales@yourdomain.com and payments@yourdomain.com)
b) You defined one account as your primary account (eg. sales@yourdomain.com) but you use the other one for your payments (eg. payments@yourdomain.com)
c) You use this in an application which integrates into paypal and the application has email security check (eg. vbulletin paid subscription API).

If all 3 above happens at the same time, the paypal payments you receive from users will go through (you'll receive the money), but application will not update user account (eg. vbulletin will not change usergroup of the payer for paid subscriptions).

This is not a fault of the application, this happens because of Paypal's changing API behaviour.

How to fix it:
Change the email address your application uses (eg. vbulletin paid subscription paypal payment address) as your primary paypal address.

This will fix the issue until the application developers comes up with a solution on coding end.

This does not affect vBEnhanced Style Purchases.

(originally posted by Logician: Paypal API changed: Your Paypal payments failing? - Admin Zone Forums)