This is why the ISPs throw up their hands at senders

I recently saw a question from an ESP rep asking if anyone had a personal contact at a particular ISP. The problem was that they had a rejection from the ISP saying: 571 5.7.1 too many recipients this session. The ESP was looking for someone at the ISP in order to ask what the problem was.
This is exactly the kind of behaviour that drives ISPs bonkers about senders. The ISP has sent a perfectly understandable rejection: “5.7.1: too many recipients this session.” And instead of spending some time and energy on the sender side troubleshooting, instead of spending some of their own money to work out what’s going on, they fall back on asking the ISPs to explain what they should do differently.
What, exactly, should you do differently? Stop sending so many recipients in a single session. This is not rocket science. The ISP tells you exactly what you need to do differently, and your first reaction is to attempt to mail postmaster@ the ISP and then, when that bounces, your next step is to look for a personal contact?
No. No. No.
Look, connections and addresses per connections is one of the absolute easiest things to troubleshoot. Fire up a shell, telnet to port 25 on the recipient server, and do a hand SMTP session, count the number of receipts. Sure, in some corporate situations it can be a PITA to do, sometimes you’re going to need to get it done from a particular IP which may be an interface on an appliance and doesn’t have telnet or whatever. But, y’know what? That Is Your Job.  If your company isn’t able to do it, well, please tell me so I can stop recommending that as an ESP. Companies have to be able to test and troubleshoot their own networks.
Senders have been begging ISPs for years “just tell us what you want and we’ll bother you less.” In this case the ISP was extremely clear about what they want: they want fewer recipients per connection. But the ESP delivery person is still looking for a contact so they can talk to the ISP to understand it better.
This is why the ISPs get so annoyed with senders. They’re tired of having to do the sender’s job.