Hi Jason,
I’d like to confirm when you say “he has hosted outside of our organisation and not in the office 365 setup”, do you mean he is not an Office 365 user?
If yes, Office 365 allows Office 365 users to view all the mailboxes and external users in your organization in Address Book in Outlook desktop client.
If your CEO is not an Office 365 user and you just want to make him receive emails sent to or from all the users in your organization, you can set up a Journal Rules. You define the recipients you want to journal, the messages sent to or from those recipients you want to journal, and where you want copies of the journaled messages to be delivered.
For detailed information, please refer to: http://help.outlook.com/en-US/140/ms.exch.ecp.LearnMoreJournalRules.aspx
If you not only want your CEO to view all the mailboxes in Outlook desktop client, but also want him to view the emails sent to the mailboxes, please refer to the following steps:
1. Assign an Office 365 Exchange Online license to your CEO.
2. Set up this Office 365 account in Outlook desktop client.
3. Give the CEO permission to access other mailboxes. Please refer to the following article for more detailed information: http://help.outlook.com/en-us/140/gg709759.aspx
4. Set up Outlook to access two or more Exchange Server mailboxes from one profile.
Thanks,
Vivian Wang