Took a look at the specification, this is what I found:

For federated servers performing delivery to a third party server, delivery SHOULD be performed asynchronously, and SHOULD additionally retry delivery to recipients if it fails due to network error.

So they should retry. Note that should is not the same as must. So there is no obligation. There is no timeline in the spec about for how long or how often retries should be done. The wording says network error.

My interpretation: the spec leaves a lot of room for implementations to differ. Network problems don’t normally last for days though. I’d guess that if your server is down for 5 minutes, you’ll still receive most or everything you’d normally receive. I wouldn’t trust on that if your server is offline for more than a day.

Check the sidebar.

It’s about self-hostable alternatives to closed online software. It doesn’t say anywhere that the hardware has to be in your own home, just that it is about self-hostable software.

Similarly, !selfhosting@slrpnk.net is about self-hosting services, the hardware part is (even with the slrpnk folk) only a prefetence.

So feel free to discuss hosting your own services on a VPS here