Problems with Custom Email Domain- SPF verification fails unresolved for months.

Since Cargo has confirmed the TXT record is published correctly and you’ve already ruled out duplicates and propagation, I wouldn’t keep deleting and recreating the SPF record.

The next useful check is whether the record is visible from public DNS, not just inside Cargo’s control panel. Apple verifies the domain from DNS during the Custom Email Domain setup. Add an email domain you already own to iCloud Mail on iCloud.com – Apple Support

If the SPF record resolves correctly publicly but iCloud+ still shows it as failed after several days, that points more toward Apple’s verification side than a normal DNS propagation delay. At that point I’d contact Apple Support and give them the domain and the exact verification error. Apple’s own iCloud Mail guidance directs non-admin users to Apple Support when the issue can’t be diagnosed normally. Postmaster information for iCloud Mail - Apple Support

I also wouldn’t remove the whole custom domain from iCloud just to retry it, especially if any addresses are already in use.

Bitte kontaktiere hierzu direkt den Apple Support: 0800 6645451 (D).