Hi, I recently came accross this issue where a customer was not recieving emails from some, but not all senders. I tested sending emails from Google and Microsoft email accounts and discovered that emails sent from Google servers were getting this error:

Your message wasn’t delivered to XXXXXXXXXXX   because the domain XXXXXXXXXXX  couldn’t be found. Check for typos or unnecessary spaces and try again.

Emails sent from Microsoft accounts were being sent and reieved fine.

I checked the DNS MX records and found that there were two set of MX records with the same priority of 10.

To fix it, I deleted the MX records that were not required, so there was only one set of records for the correct email provider.

This can also happen if the MX records priority level is set incorrectly.

If you are unsure what your MX records need to be or how to set them up, check these guidelines for Google Workspace and Microsoft

Google Workspace: https://support.google.com/a/answer/140034?hl=en

Microsoft 365: https://docs.microsoft.com/en-us/microsoft-365/admin/get-help-with-domains/create-dns-records-at-any-dns-hosting-provider?view=o365-worldwide

Hope this helps someone