By default outlook displays 12 months of mail. You can change this.
In Outlook click on file, > Account Settings > Account Settings
This opens the account settings window. In the Email tab click on Your name > Change. This opens the exchange account settings. There is a slider below Download email for the past: You can slide that to all and it will load all email in outlook.
Click Next and Ok to save that, It may take a while for it all to download, but that will cache a copy of all email.
If you login directly to Nexus365 you will also see all the mail here. There is a link to log into that on the IT services home page.