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[lklivingston]

  How do I check that?

I don't know.  It may not be something you can easily check.  One thing you can do is put the email address those come from on the list of addresses from which you want to get mail, if your email provider offers that option.  I'm trying to track this down from our end too.  I don't think it's an AO issue, but we may be able to resolve it by asking our webhost to intercede with the other internet providers to authorize emails from us.  Sometimes people tag our emails as spam, and then they start being blocked.