It appears that when DNS report looks up the MX record and does the DNS query to get the IP address of the MX record server, it's not submitting the query with a trailing .
For example (constellationhbs.com):
Only one MX record exists within the zone. This is ok, but it is a better practice to have at least two mail servers operating on a domain, to ensure more reliable mail deliverability. The MX record provided is:
preference = 0 constellationhbs-com.mail.protection.outlook.com. [54.235.159.97]
Do a lookup on "constellationhbs-com.mail.protection.outlook.com." and you get
207.46.163.170
207.46.163.215
207.46.163.247
NOT 54.235.159.97
You get 54.235.159.97 if you're missing the trailing dot on the end of outlook.com.
Thanks for looking into this!