Alias Domain emails error 550 5.1.1 but work anyway

[OldmanJohn]

I have an Alias Domain pointing to my Primary Domain. The Primary Domain has a wildcard redirect of all email to another gmail account.

When emailing anything at the Alias Domain, the sender gets an error back, saying undeliverable as the account doesn't exist, with a status of 550 5.1.1, but, strangely, the email actually gets delivered fine.

All emails sent to the Primary Domain are correctly accepted and redirected to the final destination account, so I don't think it's an issue on the Primary Domain.

How do I solve this ?

Thanks 🙂