They're checking to see whether any of the links they put in the emails are being fetched from their servers. It's stupid, but it works for most people.

I had a similar situation with SMS messages that were being sent to me with links informing me of status updates. These texts were useful, and I would go over to my real computer to check the web site. Then after a few days the text messages said "It looks like these messages aren't getting through to you, so we'll stop sending them." Which is also stupid, but it works for most people that load the web site on their phone from the SMS link. God help you if you have a dumb-phone.