iCloud+ Custom Domain Rejecting Gmail Messages – Error 554 5.7.1 [CS01]

Hi everyone,

I’m having an issue with my iCloud Mail configured for a custom domain (di******a.com). When someone tries to email me from Gmail, the message gets blocked with the error:

554 5.7.1 [CS01] Message rejected due to local policy.
Please visit https://support.apple.com/en-us/HT204137

Here’s the context:

• The domain is properly set up in iCloud+ with valid SPF, DKIM, and DMARC records.

• I ran a test via Mail Tester and got 10/10 score — no authentication or spam issues.

• The message being blocked is just a basic test email — plain text, no links, no attachments.

• The rejection appears to happen at the SMTP level, and the Gmail sender receives a bounce from mailer-da****n@googlemail.com with the CS01 error.

I’ve reviewed the linked Apple support article, but there’s no explanation for why this specific message would trigger a block. Has anyone faced this issue with iCloud+ custom domains?

• Is there a known reason for Gmail messages getting blocked like this?

• Could this be a false positive by Apple’s filters?

• Any way to whitelist the sender or check if my domain is flagged?

Appreciate any insights. 🙏

Thanks

[Edited by Moderator]