Second only to accidentally hitting reply all is deleting an important email that you still need. If you catch your mistake right away you can easily retrieve the email but a new policy being enacted by Microsoft will make sure that emails that have been deleted for over any amount of time can be restored.
Before the new policy, emails on Exchange Online would be permanently deleted after 30 days. The update will change the “Default MRM Policy” to store deleted emails indefinitely. Due to storage limitations or any other reason, administrators will have the option to make changes to their Office 365 settings including an option to permanently delete emails in the same way that happened before the update.
Microsoft’s “Office Blog” has a step by step guide to changing retention policies through the Exchange admin center.
You can either edit the name of the Default MRM Policy or create a new policy to opt out of this change. To change the policy name in Office 365 navigate to Office 365 Admin > Exchange admin center > compliance management > retention policies. Next, select Default MRM Policy, click the edit icon and then change the name of the policy.- Office Blogs
Emails are usually deleted when they no longer seem relevant but occasionally users will need to find a specific item that they have deleted. The new policy helps ensure that they can recover any lost information.