While I was studying for the 74-324 exam, I was reminded that by default the “Default MRM Policy” will be applied to a new mailbox (when you enable archiving)..
Very simply put, some items will be deleted or archived after a specific period of time. By example, files in your deleted folder will be removed after 30 days. This is unexpected for most people who are used to be able to browse their “deleted items” indefinitely.
Personally, I don’t want this to be automated because I want to be completely in control of my mail items. Especially considering the very large mailbox sizes with Office 365 (and even unlimited size for some subscription plans) I have disabled this “Default MRM Policy”.
You can set/disable the retention policy either using PowerShell or using the web interface by going to the mailbox, details, retention policy and selecting no policy.
Also if you want to know more about archiving, take a look at a couple of blog posts created on Techdom.nl by my colleague Dominique Hermans.
PS:Also take a look at this great post discussing backups, recovering deleted items and why you might want to enable litigation hold: http://www.spaceage.co.za/blog/technical/office-365-backup-data/. Also take a look at this article covering single instance recovery: http://help.outlook.com/en-us/140/hh125820.aspx.