Hi Jan,
Thanks for your feedback.
Are you using the default retention policy? If so, when delete a message, it will be stored in Deleted Items folder for 30 days.
Later, it will be moved to Recoverable Items folder. By default, the retention period for deleted items in this folder is 14 days. After 14 days, the message is automatically moved to the Purges subfolder, which isn’t accessible to and can’t be recovered by the user. Only an administrator can recover a purged e-mail message. Administrators can use Multi-Mailbox Search to search for purged items. For detailed information about Multi-Mailbox Search, please refer to
help.outlook.com/.../cc511392.aspx
When the retention period for an item in the Purges subfolder expires, the item is permanently removed from Exchange Online and can’t be recovered by an administrator.
For more information about single item recovery process, please refer to
help.outlook.com/.../hh125820.aspx
Based on the current situation, please use Multi-Mailbox Search to search for those emails. If they cannot be searched, they’re removed permanently and cannot be recovered.
Besides, if you want to store some import messages, I suggest you enable Archive feature of the mailbox.
help.outlook.com/.../ff628726.aspx
Thanks,
Young Yang