Ref T9031. This isn't a complete fix, but makes mail stamps work better for subscription changes. You can now route these on the client, and after T13069 resolves you should be able to route them on the server.
Basically, these three changes are all a "mention" of @dog:
alice added a comment. @dog, can you look at this?
alice added a subscriber: dog. Dog, can you take a look at this?
alice added a subscriber: dog.
The last one is a little flimsy, but I think the implied intent is always @dog, heads up on this.