Although mailbox data in Office 365 is fairly secure, it does not mean it is protected from deleting. Users, either by accident or on purpose, can remove their emails from Inbox and Deleted Items folder or even purge them from Recoverable Items folder. What’s more, retention policies can exist, which live only for deleting certain emails from users’ mailboxes. Of course, Office 365 gives you a plan B, just in case you would need to recover deleted emails. This plan, however, only works when the time gap between deletion and recovery is not too big.

more details:

https://www.codetwo.com/admins-blog/how-to-recover-deleted-emails-in-office-365/