Upon further investigation, the sending email seems to have received an email from Microsoft mentioning:
Delivery has failed to these recipients or groups:
[my receiving email used in testing]
Your message wasn’t delivered because the recipient’s email provider rejected it.
Diagnostic information for administrators:
Generating server: […..].PROD.OUTLOOK.COM
Remote server returned ‘550 5.7.708 Service unavailable. Access denied, traffic not accepted from this IP. For more information please go to https://go.microsoft.com/fwlink/?LinkId=526653 AS(7230)
After checking the Microsoft link and the info for further details it mentions:
5.7.708 Access denied, traffic not accepted from this IP
This error can happen when you are trying out a Microsoft 365 trial tenant. If you receive this error before you can purchase licenses, contact support to request an exception for the low reputation IP address until you’re able to purchase licenses.
https://learn.microsoft.com/en-us/exchange/troubleshoot/email-delivery/ndr/fix-error-code-5-7-700-through-5-7-750#57708-access-denied-traffic-not-accepted-from-this-ip
Indeed, this MS 365 is a new account and the client is actually still using the trial version.
So it seems that this could cause issues like that