Hello Bcampe,
Thanks for your feedback.
I fully understand that you have concerns that the issue occurs after changing email service.
As I mentioned, there are no record shows that the emails have been sent to Office 365 currently. As a result, we need to confirm whether the emails have been delivered to Office 365 successfully.
If the sender email system support (here is Rackspace) confirms that the emails were delivered to Office 365 successfully, we will have a further investigation to find the root cause to fix the issue. Thanks for understanding.
Thanks,
Claud