Ok let me revisit this.
My client is hosted by YAHOO SMALL BUSINESS. Yes, I know they are the devil. I have no choice here.
I have tried using JetPack alone, with no luck.
I have tried adding WP-Mail-SMTP plugin, also no luck.
I've tried variations on all the settings available to me, no dice.
I've tried setting up a gmail account for him to send to - nothing.
I hear that Yahoo won't let you send emails "from" anywhere else but your domain - I've tried playing with that idea, also with no result.
HE (the client) has told me that he "can't send email to himself" - which sounds crazy to me. THAT might be the problem? I watched him send an email to himself, and it never arrived. He says it's always been that way. So maybe the contact forms are working but Yahoo is blocking all my best efforts?
I am losing my mind over this. I'll try another plugin (Contact Form 7 perhaps), but given all the mucking about I have done, and no emails to show for it, I have very little hope.
M