When migrating from Hosted Exchange to Office 365 I needed to migrate the mailboxes of users. As the number of users were limited I decided to do a quick and dirty export / import using a .pst file. After the fist mailbox I did a quick check to make sure all data had been migrated correctly. I noticed that the mail only went back one year. It turns out that by default the offline mailbox retention period is set to a year. To do a complete data migration you need to change this to all mail.

To change the retention period follow these steps:

• Open Outlook.
• Click File.
• Click Account configuration.
• Click Account configuration…
  

  Open Account settings

• Double click the email account you want to change the retention period of.
• Move the slider of the Offline settings all the way to the right
• Click Next >.
• Click complete.

That’s it. Give it some time to synchronize all the data locally.