What is needed is an index for all the instances that are willing to be published. All server instances would have access to the public index via some gossip protocol.

Also are there any other server instances other than the php server? Seems like they ought have used the matrix protcal instead of active pub. More server implementations in different langs, and it has e2e build I. Don't think actpub has that.

Well, sure - if someone wanted to be the hoster for all mail traffic on planet earth, then they would need a lot of resources and a business model to finance it.

But if that is their plan, why would they have used ActivityPub in the first place?

So I guess they could just shut their door as soon as the traffic is too much for them to handle. And tell people to look for another host or host themselfes.

right now. Let's make it grow