if you delete an item in O365 mailbox, it goes into "deletem Item". This system folder has got a limited retention period to allow to "restore" an item until 60 days (seems to me)
My company provide a backup service of all mailboxes without retention for all folders, but in this case, if i'll not apply a specific retention for "Deleted Item" folder my repository will grow up infinitenly, because the backup run one time per day, so, even if O365 will delete 60 days older trash's item, in my backup will be stored forever. The maxium size for a mailbox in O365 is 50 GB, but backuping all folders, including "Deleted Items", without retention will use more than 50 GB in the months / years, and that's a problem.
Please let me know if i was not clear, my english is not good.
Thanks a lot, Niccolò Freddo