Clutter is a feature in Outlook 2016 that moves some mail to a special "Clutter" folder. Outlooks moves emails you are least likely to read into this folder. It does this by keeping track of which emails you open and which emails you ignore.

To turn Clutter off and prevent it from moving emails, follow the instructions here: https://faq.oit.gatech.edu/content/how-do-i-turn-onoff-clutter