Internal domain mail hitting junk mail folder - WHY?

My webmail provider, our IT contractor, and I have been trying to figure out why inter-domain email winds up in junkmail folders in Outlook 2010. All senders are set up as safe senders, they are set up as contacts in Outlook, and our domain is set up as a safe domain. Two locations who receive email as junk are using Avast anti-virus. The other two locations are using Eset, and they have no issues. So it MUST be an Avast problem. HELP!!! This has been going on since JANUARY, and I am getting my butt chewed because it is still going on.