We just found an email from Google which is about AdSense Payment Notification, not sure when they started to send this kind of email, but maybe our AdSense earning is small, we never found the similar content before.
You’ve been issued a payment from Google AdSense
Please note that we’ve recently issued a payment to you for your AdSense earnings. Your payment represents the total of your finalized earnings accumulated through the end of last month.
For more information, please log into your account and visit your Payments page. Depending on your form of payment, you may also be able to find a reference number there to track or pick up your payment.
Congratulations! We hope you receive your payment soon.
The Google AdSense Team
Normally you should receive their email notification at the end of each month, as long as your payment is qualify to send.