If a server seems to be unreachable, Gnus will mark that server as
denied. That means that any subsequent attempt to make contact
with that server will just be ignored. “It can't be opened,” Gnus
will tell you, without making the least effort to see whether that is
actually the case or not.
That might seem quite naughty, but it does make sense most of the time. Let's say you have 10 groups subscribed to on server ‘nephelococcygia.com’. This server is located somewhere quite far away from you and the machine is quite slow, so it takes 1 minute just to find out that it refuses connection to you today. If Gnus were to attempt to do that 10 times, you'd be quite annoyed, so Gnus won't attempt to do that. Once it has gotten a single “connection refused”, it will regard that server as “down”.
So, what happens if the machine was only feeling unwell temporarily? How do you test to see whether the machine has come up again?
You jump to the server buffer (see Server Buffer) and poke it with the following commands:
gnus-server-deny-server). This will effectively disable the server.
gnus-server-copy-server). This can be useful if you have a complex method definition, and want to use the same definition towards a different (physical) server.