The new Outlook for Windows will allow opening and saving classic email attachments offline, starting mid-May 2025 rollout. This requires enabling offline mode and attachment syncing in settings and depends on the existing OWAMailboxPolicy OfflineEnabledWin configuration. OneDrive and SharePoint links still need internet access.
Applies to: Exchange Online
Source: mc.merill.net — data via Merill Fernando's open archive (MIT).