Your main premise seems to be that the recipient should take all this very personally. But it’s not personal, these are businesses, discussing a business related topic.
The VAST majority of business to business communications that involve attorneys don’t go anywhere near an actual filed lawsuit.
Like I said in my original reply here, we are talking about sending threats to third-party isps, with which the originator of the smtp traffic has no existing business or contractual relationship.
I would think that if you were the ISP owner blocking our mail to your customer, a single phone call from the customer or from us would convince you that our IPs were not a spam threat. That's certainly the way I prefer to deal with anything. But when dealing with something as monolithic and totally deaf as AT&T, lawyers were the only way.
Of course there’s a lot of garbage out there too.
Make it a legal and business decision instead of a technical one. And find an audience that knows the practical benefits of quick resolutions to reasonable requests.
And I say this as a person that would likely respond in a similar punitive and/or principled way to what you described upthread. Which is exactly why I leave it to the lawyers.
I'd go further and say that choosing to unblock some IP addresses out of a block you don't like is always going to be a business decision. The whole rationale for blocking in the first place was that a sender is untrustworthy.
Untrustworthy senders generally don't have a team of lawyers, and if they do (and they're actually sending spam) you can show what spam they sent and keep them blocked.
All the legal letter does is force someone who's otherwise too busy to look at this particular case for the 5 seconds it takes to determine that it's not a threat. Sad to say that being head of IT for a company for a decade doesn't get you the same level of respect; it might if they even bothered to open your email, but there's something about postal and legal letterhead.
It doesn’t mean they are thinking to sue you. Not even close. When you actually get served, that’s when they’re thinking about it, because actually serving you is a few bits of paper and requires a few brain cells. Sending you what amounts to a form letter? No brain cells required.
The implication is that if you do not do whatever is demanded in the letter, the next step will be the client of said lawyer escalating the situation to paying their lawyer to actually sue you.