It would be very useful if Proton Mail allowed users to mark specific folders as always available offline. The app should automatically synchronize and maintain the latest version of all emails in that folder, including all attachments, so the complete folder contents are accessible without an internet connection.

A practical example would be traveling: before a stable internet connection is available, users could already access all important travel-related information such as booking confirmations, tickets, or reservations. This would also be valuable when users know they will be in a location without connectivity for a period of time. They could rely on having the latest synchronized state of at least this selected folder available offline at all times, including all attachments.