Can't send an email to one of my customers, it stays in the outbound queue trying to connect.
After 2 days it would time-out with 4.4.7 Error.
The customer recently changed their mail system to use mycustomer-com.mail.protection.outlook.com.
However, they still have the old Mx record published as well mail.mycustomer.com which doesn't even work.
Here is my main concern: When using the NsLookup (set q=mx) from my in-house DNS server (which is connected outside with my ISP dns servers) I can only see their old Mx record. When testing with a different dns public server (Google 184.108.40.206) I actually see both Mx records.
I checked my firewall (router) and DNS queries on port 53 are not getting blocked.
Other hosts seem to be resolving fine.
I also removed the old pointer in my dns cache for this specific domain, but it still comes up with old Mx record/IP address only.
I am not sure what to test and do at this point; I did ask them to remove their old MX record, not sure if that will resolve my issue. They also told me that I am not blacklisted on their new server (not blacklisted in general).
Thanks for your help, much appreciated.