Cox not receiving emails from Microsoft (hotmail.com, msn.com)

For a message sent from a msn.com account to a list that includes multiple cox.net addresses, eventually a non-delivery notice comes back saying all the cox.net recipients were unreachable.  The message for all of them is exactly the same:

<username-deleted>@cox.net
4/3/2019 6:55:08 PM - Server at BN3NAM04HT178.eop-NAM04.prod.protection.outlook.com returned '550 5.4.317 Message expired, cannot connect to remote server(421 cxr-ibgw-6003a.stratus.cloudmark.com cmsmtp 40.92.10.85 blocked. Refer to Error Codes section at https://www.cox.com/residential/support/email-error-codes.html for more information. AUP#CXCNCT)'
4/3/2019 6:49:13 PM - Server at cox.net (34.212.80.54) returned '450 4.4.317 Cannot connect to remote server [Message=421 cxr-ibgw-6001a.stratus.cloudmark.com cmsmtp too many sessions from 40.92.11.96. Refer to Error Codes section at https://www.cox.com/residential/support/email-error-codes.html for more information. AUP#CXCNCT] [LastAttemptedServerName=cox.net] [LastAttemptedIP=34.212.80.54:25] [SN1NAM04FT032.eop-NAM04.prod.protection.outlook.com](421 cxr-ibgw-6001a.stratus.cloudmark.com cmsmtp too many sessions from 40.92.11.96. Refer to Error Codes section at https://www.cox.com/residential/support/email-error-codes.html for more information. AUP#CXCNCT)'

I cannot get individual messages to my cox.net account from various Hotmail.com and msn.com accounts.  I expect I'll get non-delivery notices for these in a day or two.

Something is broken.  How do I get this message in front of someone who can DO something?  Cox Chat and Tier 1 phone reps were typically useless.  ("What device are you using?  What is your operating system?")  THIS IS A MAIL SERVER PROBLEM.  PLEASE HELP.