@Patrick Loh Thanks for your patience on this.
As discussed over the offline, I am summarizing our discussion and posting as an Answer,
Issue: Customer is using WordPress on Azure app service with Email integration and when they are trying to an email from the WordPress it is failing with the error message "Sender domain xxx-xxx is not linked to this Azure Communication Resource."
Resolution: Upon troubleshooting further, we see that customer has connected his communication service to custom domain (email service) and in the app service connection string they are referring to another Azure managed Domain that they have manually created. Due to this misconfiguration email are getting failed with above error message.
I hope this is helpful, please let me know if you have any further queries.
Please accept as Yes if the answer provided is useful, so that you can help others in the community looking for remediation for similar issues.