My main concern with this is I don't know how well activity pub can scale, and we've already seen various interop problems between different types of platforms. And if email is any indication, once activitypub gets popular it will NEVER GO AWAY and all the developers will hate it.
First inter-user messaging was 62 years ago, first email client was 51 years ago, first spam was 45 years ago, first attachments were 31 years ago.
I don't work with it directly, but it seems we've bolted on a lot of "fixes and functionality" onto email, but the underlying protocols haven't changed dramatically.
And now nearly 56.5% of all emails were spam last year. Though I don't think that counts intranet spam I get from my bosses all the time.