messages blocked

There must be something about these emails that is causing the iCloud email servers to reject them. Do you know if they contain attachments? It may be because they are too large to be accepted. However, the error seems to indicate that this issue relates to mass mailing.

I have the exact same issue... I have a gmail account and an iCloud account. I'm forwarding all my gmails to iCloud. I had ni issue until December / January when this message appeared.

One of my friends (who's using a forwarding from gmail to icloud too) has the exact same issue...

FWIW. I also forward my Gmail account to one of my iCloud accounts and do not have any issues sending or receiving messages.

The error that the OP is getting appears to be related to their iCloud email servers rejecting bulk mail being sent from their Gmail account per the policy set on those servers.

If by "no" you are confirming that the issue is only with these Gmail messages are being rejected by the iCloud email servers, then it must be something about these messages that do not meet the policies set by those iCloud servers. Again, the link in the error indicates that this may be caused by receiving bulk emails from the Google account.

This error indicates that Google Mail was not able to send an email to your iCloud email account. Are you having any issues receiving any other emails, whether they are from Google or not?