They are very different things. These digital mailboxes are exactly that, just a digital mailbox. They allow you to receive secure letters digitally that would otherwise have been sent in the post. Private companies can also send you mail using them. Altinn is basically how you interact with the Norwegian government online, fill in forms, submit changes, and all kinds of things. It’s only used by government agencies. The other key difference is that the digital mailboxes are entirely optional, whereas it’s very difficult to function in Norway without using Altinn to communicate with the government and its agencies. I hope this helps.