ProtonMail uses a flexible hybrid system of folders and labels to maximize productivity and organization within your inbox.
ProtonMail currently uses 6 folders to hold all of your emails – Inbox, Drafts, Sent, Starred, Archive, Spam, and Trash. Emails are moved across different folders when you use the different organizational buttons in the ProtonMail interface. For custom folders, ProtonMail supports Labels.
To create a label, use the Labels tab in settings or the Label Settings inside your ProtonMail inbox. A Label can be used to emulate a Folder by applying a Label to a message and then Archiving it at the same time. This will remove the conversation from the Inbox, and put it under its own Label “folder” in the sidebar.
You can add labels to any email regardless of which folder it resides in using the Labels drop-down.
Please note: Labels are not stored in the label tag, they are stored in the folders (Inbox, Archive, Spam ect.). If you delete the message from the folder, the message will be deleted from the label as well.