Hi Jeremy,
I tried this now on a stock WP4.0 installation without any plugin but JetPack and only Contact form activated and I am still getting the same problem. The installation this time is within a subdomain wp.domain.com and the email sender is wordpress@wp.domain.com
Would it help if I'd setup an admin account for you there so you could have a look?
Btw here is a screenshot of the providers maillog showing the error:
https://www.dropbox.com/s/r1xkvse3rdvg26h/Screenshot%202014-10-09%2020.40.59.jpg?dl=0