Delivery notification, delivery has failed - from iCloud email

I have my own domain which comes with email accounts. When I try to send a test email from my iCloud email account to this new email account, I get a postmaster delivery notification to say delivery has failed:

This happens whether I send from iOS device or Mac. Yet if I send the same test email from my gmail account the email sends and receives without any issue. And I have received emails from other people okay:

The issue only rises from my iCloud account.

I have just moved this domain from one provider to another. Is there so,e cached data somewhere causing it to stall? The MX record is pointing to the correct mail server. And I have received mail from other iCloud users, it’s only my own that won’t send.

Hope someone can help!