Requirements for SMTP sending when using out-of-cox domain "from" address

About a month ago my Thunderbird email client started receiving error messages when sending email through cox SMTP.

I am sending using a "from" address other than our cox adress (at another domain).

Sometime back I setup an SPF record in my domain zone file to support this and it has worked well until a month ago.

Here's the relevant message regarding the error:

CXSNDR
There was a problem with the sender's domain.
Your email failed authentication checks against your sending domain's SPF, DomainKeys, or DKIM policy.

I can't find any detail on these requirements.

Can anyone tell me the *exact* requirements for this setup?

Thanks