"Undisclosed recipients" is often placed in the "To:" line by some mailers when the email being sent has no entries in the "To:" or "Cc:" lines. The sender has used the "Bcc:" feature of email to send the email to one or more people, without revealing who they are.
So how do you find out who they are?
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You don't.
That's what "undisclosed" means. The information about who the email was sent to is not included in the email. There is simply no way of determining if it was sent to anyone else and if so, who.
Now, to be complete, I do recall hearing about some old email programs - and we're talking ten or twenty years ago - that got the whole concept of "Bcc:" and undisclosed recipients wrong. They included the Bcc'd recipients in headers that everyone could read if they knew how. But that was a serious bug and has long since been resolved.
Today's email programs simply don't disclose "undisclosed recipients".
It would be wrong to do so.
