DKIM check keeps failing on Namecheap too

Good instinct to mention the registrar — the underlying cause is often the same host-field issue we walked through here, and Namecheap has a similar quirk to GoDaddy: their “Host” field also auto-appends your domain in some record types. Can you double check whether the Host field has your domain typed in it twice? Just enter default._domainkey with nothing after it.