Proofpoint Blocking Ups Clients

I host MIAB (Mail in a box) on DO (Digital Ocean)

Received an email from someone sharing this hosting service saying that he cannot email any UPS addresses with the following error:

<xxxx@ups.com>: host mxa-002b8001.gslb.pphosted.com[x.x.x.x] refused to talk to me: 554 Blocked - see https://ipcheck.proofpoint.com/?ip=x.x.x.x

Does UPS feel that it is acceptable to lose customers because of this?

This is not an isolated incident as I found out from a quick online search. These are customers that are being blocked is this behavior even legal?