I'm not saying "yes, you currently can do this with the activitypub protocol as it is," I'm saying this feature could be added to activitypub, and I've made specific references to protocols like POP and IMAP that handle logging into email servers from various client applications. I'm not going to code it myself, I'm an attorney, but I do know enough about computer science to know that there is no computabilty issue with my proposal, and that you dislike it primarily because you don't currently have an idea for implementing it, which is not my concern at all.