Becky, please check with Alex (Cox support rep) from Sunday afternoon, look up my ticket and confirm (or not) what he told me about email limits and Cox internal documentation for residential customers now in effect.
I am using Outlook 2016 (Office 365) with Windows 10. Every Sunday I send a church announcement to about 30 members. It's been working fine for years. Yesterday, I received the bounce error message " ... 552 5.2.0 ... internal error" for each intended recipient.
During my own testing I reduced the email to 15 email recipients (same error), then 10 (same error) and then finally 9 (no error). Finally I broke the message into multiple "groups" of 9 each and succeeded in sending my original 30 members the email.
My security settings are correct and match Cox's recent changes.
It's not realistic to require customers to use your online Webmail app or export all my address book to it. Outlook 2016 is widely used and respected email "utility" with many features.