By default, you can recover deleted items in Office 365 for 14 days. However, you can put a litigation hold on the mailbox which would increase the recoverable items up to a storage capacity of 30GB (Assuming you have Exchange Online Plan 2). And this doesn’t count against your main 25GB mailbox. Follow the steps below to recover them in OWA and your Outlook client.
Retrieving Deleted Items in OWA
Login to OWA and right click on your “Deleted Items” folder.
Select “Recover Deleted Items…”
Right click on an email and select “Recover.”
Retrieving Deleted Items in Outlook 2010 and 2013
Click on the Folder tab and select “Recover Deleted Items.”
Highlight the email or emails you want to recover and hit the recover messages icon.