Windows Office Hours: September 28, 2023

My comment was deleted with no information about why. I'll try one more time... A customer is facing an issue with emails sent to "customername.onmicrosoft.com" email addresses. They want those emails sent to such aliases (default email addresses are using customer's custom domain) to be routed to a 3rd party security solution. Microsoft official response was to have them routed through and ETR (Exchange Transport Rule), which never worked. We found later the reason is because transport rules apply after the recipient is resolved and the post-resolution email address is changed to the user custom domain bypassing the rule. We have spent many many hours in calls with Microsoft Support without a fully working solution. We don't want the 3rd party solution to be configured by Microsoft, just all traffic sent to "customerdomain.onmicrosoft.com" email addresses routed to it. This was easily done for the custom domain (pointed the DNS MX record to the 3rd party gateway), but there is no such option to the same for the "*.onmicrosoft.com" domains (although Microsoft owns these, onmicrosoft.com subdomains are independent from its parent and it should be possible), only TXT records can be managed for them. Anyways... I was hoping to find any help about this on this session. Any help would be great! Thank you!