Apple mail has the ability to categorize incoming email into four broad categories. These are Primary, Transactions, Updates, and Promotions. If you do not want this feature turned on, see the following steps.
1. Open iPhone's mail app on your device
2. If you are not already, click into an Inbox
3. Click on the dot dot dot in the right corner
4. Click on List View
You can toggle your preference back and forth: Categories or List View.