Microsoft has changed its data retention policy for Office 365 email and the upshot is that deleted items will never disappear, unless you set up a policy to bin them.
Exchange Online currently deletes things in the Deleted Items folder after 90 days. Redmond's pitching the new policy, explained here, as a hedge against irritating accidental deletions.
Microsoft says the never-ever delete policy will be imposed on anyone that hasn't set up a custom messaging records management (MRM) policy. Any custom policies that admins have set will remain in force and override the never-delete regime.
The change to perpetual storage is challenging: plenty of organisations are happy to have deleted emails definitively disappeared. Plenty of others will be happy that automatic deletion has ended, as many jurisdictions around the world operate under laws controlling the spoliation of evidence. Those laws can mean that deleting emails is a no-no in case they're needed for future legal action.
Microsoft's change may well have been made with such laws in mind: Redmond probably doesn't fancy the chore of jurisdiction-by-jurisdiction policies or lawsuits resulting from its deletion of critical messages. Or it may just have wanted to add flexibility to Exchange Online.
Either way, the change looks like something admins need to consider because some Office 365 bundles directed at businesses come with inboxes limited to 50GB of storage. Storing deleted emails forever might turn out to be a very expensive proposition. ®