If you receive an email from PhotoKeeper just delete it. The app is a ploy to get you to install their App on your mobile device. Once installed the app will harvest your contacts and send them the same type of email you received, without your knowing. Why would it do that? I’m guessing the app was designed to gather data, email addresses, and personal information for sale by the company who is collecting the data. This type of app is essentially malware that works like a type of computer virus called a worm that works its way from one system to another. Below is a photo of one of the emails I have received.
Being skeptical I did not follow any links but researched instead. Despite the positive reviews in the App Store the full list of reviews is peppered with folks proclaiming the app as malware. Heed their, and my warning and just delete the emails you receive.