Mistakes is something that we all do every day. In case you delete some important email that was placed in your trash folder, typically you shouldn't be able to recover it. Thanks to Cloud Mail and the Dumpster folder, you can recover the email yourself within 30 days. The deleted emails are accessible by right clicking on Trash and selecting "Recover Deleted Items". Simple, isn't it?
Also you can set a retention policy on any folder you like. You just need to right click on each folder and select the retention policy you wish. This option is available under folder's properties. By enabling message retention on a folder, messages which fall within the retention range you selected will require explicit confirmation before being deleted.
In addition you can enable the message disposal option if you like. That way, messages in the particular folder which are older than the disposal threshold you set will be subject to automated cleanup and deletion. So you can keep your mailbox clean from old emails without needing to delete them manually.