Offline Access

You can still access your mail offline if you have an app on your phone or tablet.

The New Outlook does support some offline content it has recently been launched and is still rolling out to some customers. Please note that this is a relatively new feature.

https://www.microsoft.com/en-us/microsoft-365/roadmap?featureid=178030 

For details on how this works, please view this Microsoft guide: https://support.microsoft.com/en-gb/office/work-offline-in-outlook-2460e4a8-16c7-47fc-b204-b1549275aac9