We've had a similar issue come up, our solution/problem was similar to Cathy Danahy.
Another issue I've seen is if you are using a shared or routed email box (one email address - but routes to multiple people), those can cause spam issues as well until your IT department white lists. Another way to narrow down some problems would be to use personal emails versus corporate. I have several test accounts to test things out with different email providers (yahoo, gmail, outlook, aol...)