Q: (from Megan)
It’s my understanding that there is no way to restore an Exchange Online mailbox back to a certain point in time natively in Office 365 and that you would need a third party tool for this. Is that correct?
Exchange natively is set for 14 day retention for deleted items. If you choose, you can extend the retention and administrative recovery to 30 days. If you want to support recovering message beyond 30 days, you would need to use a 3rd party service for the backup.
Backing up email in Exchange Online: https://technet.microsoft.com/en-us/library/dn440734(v=exchg.150).aspx
Recover deleted messages in a user's mailbox: https://technet.microsoft.com/en-us/library/ff660637(v=exchg.160).aspx
Change how long permanently deleted items are kept for an Exchange Online mailbox: https://technet.microsoft.com/en-us/library/dn163584(v=exchg.160).aspx