What I will have a problem with is sending you headers from an email that failed...because there's no email
Understood. But you can add me in to the to: or cc: field and re-send.
I could send you headers from one that *did* go through
Nope, don't want that one ;-)
Oh...as for that, I'm using the built-in DH mail servers.
Ok. Well I can't verify it without your domain name, but you seem pretty tech-smart, so I'll trust that you've set that right.
the email is being sent from WP using a DH-hosted email account to another DH-hosted email account...with what should just be a different reply-to email.
So you see the reply-to problem with DH -> Gmail and DH -> DH? So weird. I've never setup a contact form that didn't have the reply-to set as the user-supplied address.