Email Bounce issue with code 550 but super weird...

I can initiate a email to the recipient and they can respond, but I can't reply to the response.  I get a code 550.  Nor can I reply to email if initiated by them. This doesn't affect recipients that also have cox as their internet server.. I have tested it using direct email from Cox website to isolate whether it is an Outlook issue or Cox issue.  It is definitely a Cox issue.  I have spent several hours on the phone with Cox after a useless time with the chat help and they can't figure it out.  If it was the recipients server issue, they shouldn't have even been able to receive the initial email generated by me.  It's only bounced once I try to reply to their response.  I also noticed that if I cc another individual it will bounce both even though their servers have nothing to do with each other.  I'm set up on Pop server, recently changed it from IMAP and it occurs under both set ups as I recall. One thing the recipients have in common is they are businesses with their own business extensions.  One uses Blue Host and I can't recall what the other one uses.  It's really annoying not to be able to have a back and forth exchange with a vendor that I'm doing extensive business with.

Any thoughts?