Thanks for the reply Pioneer!
When I spoke to Google's gmail team (who are the email provider for my domain) they remote-viewed the SMTP settings I had in the plug-in, we tried a couple and they checked through the error messages I was seeing. They were pretty helpful.
Do you have any suggestions what I should be looking for with either the email provider or the web host - any common issues?
They both tell me that my DNS is correctly setup, so I'm not sure what else to look for.
Thanks again!