Order emails not sending to particular email addresses

[RSHV16]

Hi All,

We have a shop that we run for a customer.
He recently switched his domain name, the previous domain name and the new one are all running through the same hosting: BT

The difference is now he is not receiving the order emails that come through. They use Microsoft 365.
If I change it to our email address, which runs through Google Apps, I receive these fine.

Any ideas on why the order email isn't coming through?

Thanks in advance,
Rowan
Pople Media Solutions