It doesn't matter if I seize the server, hack into the server, eavesdrop on the message traffic, steal an end user's laptop, etc. The only way any message will be decrypted and readable is when any end user logs in and allows his private key to be used to decrypt his messages. Nobody else can read them ever, period, end of story.
So let's accept the fact that a cache of encrypted messages from a server or central laptop cannot be decrypted. One thing that can be done with them is see who sent a message to whom, when they sent it, and how large it was. There is no encryption of mail message headers. The To, From, Date and Subject are all cleartext. If people are sloppy they might reveal things they should not reveal in their subject line.
Another possibility is that the messages were not encrypted. It might sound cool on a Q thread to say the messages were encrypted. Makes it sound more sinister. But they may not have bothered encrypting the message traffic. Sloppy, lazy, whatever, but it would not surprise me at all.