So we have many issues with the sub-domain that we are using with the marketing cloud. One of them is: If we want to reply to the emails from Outlook, we get an error message that says:
Undelivarable and the content is: reply-fecc157376670c7c-2139_HTML-104564539-100025066-1 wasn't found at hello.ourSubdomain.com.
Also every time a client replies to one of our emails, @hello.ourSubdomain.com is getting appended at the end of their actual email address.
We did whitelist our IP address and this was resolved for 2 days. But later, we started getting the same errors.
Best Answer
Have you set the MX entries for Marketing Cloud servers?
I think you need to use the localpart "reply" of your subdomain. So in your Reply Mail Management you need to set the domain like: reply.hello.ourSubdomain.com
https://help.salesforce.com/articleView?id=mc_es_dns_record_maintenance.htm&type=5