A couple of hints.
1) It is illegal to send to a mailing list without offering an unsubscribe from the list option.
“Every marketing email you send must give the person the ability to opt out of (or ‘unsubscribe from’) further emails.”
We might hope that a law firm knew this, you know, the law.
2) BCC exists for a reason. Find out what it is
There are 405 addys in that first line of the original email. I have, of course, just responded to all……