IT came back saying that they have a regional block for the US. Is there any official documentation which Lets Encrypt server IP addresses must be allowed for validation?
The official documentation explicitly tells you that they won't tell you the IP addresses for the validation bots. They are hosted on several cloud providers and might change without prior warning.
Right now, there should be four (might change) of which three (might change) are in the US (might change) and one (might change) is in Europe (not sure if NL or DE) (might change).
If you want to block http from the US, but you can keep your DNS nameservers reachable, you can use the dns-01 validation method instead.