Solution that worked:
I recently had an issue on an Exchange 2010 server that had anti-spam measures enabled. The owner of the company could not get email from a charter.net email account even though I had whitelisted it on the Exchange 2010 server.
It turns out that the owner had accidently blacklisted the email address on his local Outlook 2007 client and Exchange actually reads that blacklist and rejects the message (even though you whitelist the message on the server).
Once I removed the email address from his Outlook blocked senders list the charter.net account was successfully able to send him email.