smtp issues with cox.net (or any blocking relaying)

cox.net does not allow me to send from their network using zoho mail (zoho accesses smtp server from outside the cox network)
I can get my cox account incoming mail from inside cox network or outside
If inside zoho mail can get but not reply
If outside zoho can get mail and can send new mail (from zoho mail default acc), but I cannot reply to cox account mail using zoho when I am outside.
I ned to be able to receive cox mail (that's OK) and use the Zoho mail acc (default) to reply. When i am in cox mail folder and I reply to cox mail the only acc I see in the pulldown is cox, no zoho.
Ideal solution is to have the smtp of cox account configured to use smtp of zoho. How can I do that? Preferable the reply to will remain that of the cox account.
thanks
Marco