Trouble getting emails sent via Apple Mail AND via my email.cox.net

I am having difficulties sending emails to certain (not all) parties.  Oddly, it mostly seems it affects friends living in TX and PA.  I often will get the following pop-up when I hit “Send”  - but NOT always:

! An error occurred inside the server which prevented it from fulfilling the request.

No other direction or detail is provided to trouble-shoot.  This error message does not always show up and I only know that a party has NOT received my emails after days go by without getting a response (and I get confirmation from them later that they never got my email).

I use Apple Mail and updated my mail server settings earlier in 2018 per Cox’s direction & specifications.

For incoming I have mail set to imap.cox.net / Port #993 using TLS/SSL

For outgoing I have mail set to smtp.cox.net / Port #465 using TLS/SSL

My OS is Sierra 10.12.6 and I diligently install all security/software updates Apple posts for Safari and my computer.

I’ve even deleted my username & password and re-entered them in both my in-coming & out-going email server settings per one of the Forum’s suggestions to another customer with a similar issue - but that has not resolved my problem.  My trouble sending emails described above also occurs when I use myemail.cox.net as a work around.  Any advice is much appreciated.