[ntype]

the key term here is "connection to your server has failed"

I'm putting my money on the server being down.

And you could lose it all...
There are plenty of reasons why the connection to the server has failed. Could be that the computer couldn't reach the POP server (imagine some router down), The computer having an firewall who blocks certain responses, etc.

Server is "down" should be considered among other options (and like if his isp is serious he's using postfix or some variation and you could almost consider that the server is down option is not to be considered)