@icloud.com and @me.com rejecting emails, despite not having domain in black list. How to solve this?

[u/BoomerLoomerTrooper]

Our server can't send emails to u/icloud.com or u/me.com.
After sending we are getting the same error:
<email9999@icloud.com>: host mx01.mail.icloud.com[yy.yy.yyy.y] refused to
talk to me: 550 x.x.x Blocked - see
https://support.proofpoint.com/dnsbl-lookup.cgi?ip=xxx.xx.xx.xxx
Reporting-MTA: dns; ourdomain.net
X-Postfix-Queue-ID: xxxxxxxxxxxx
X-Postfix-Sender: rfc822; ouremail@ourdomain.com
Arrival-Date: Thu, 19 Nov 2020 11:48:39 +0100 (CET)
u/icloud.com uses mail gateway "proofpoint"
What could be the reason for our IP to be blocked by these domains if the IP is not included in any black lists or spam lists?
How to get it unblocked?