Rules can be set for managing your email messages automatically. Most commonly this is used to send certain messages to a particular folder.
In the event that you want to setup rules to automate sorting of message, you can do so from the rules in O365 mail. Additionally, if you find messages are 'missing' and not in Junk folder, it may be worth checking your rules to make sure more messages than intended are being automatically managed for you.
To check your mail rules, first open your email and click the settings gear box in the upper right. Then at the very bottom of the menu that pops up, choose "View all Outlook Settings":
In the all settings menus if you choose "Mail" and "Rules" you should be shown a list of all your currently active rules, including an option near the top to add a new rule.