Forwarding problem with aliases

I've got a sendmail server running that needs to forward a couple of addresses to other destinations.  I've set these up in aliases.  Most of these work but one recipient has an earthlink account and it rejects the forwarded message if the sender is not local to the sendmail server. The error message in the returned mail says "The sender's address is rejected for policy reasons."  Kind of cryptic.  The end user whitelisted the server's domain but I don't think that is where the issue is.  I think it is because the sender could be from anywhere and that doesn't match the domain of our sendmail server.  But that is just a guess.  If I send mail from an account on the server to the one of the forwarding aliases, it forwards to the earthlink account just fine.
Anybody know anything about earthlink?  Any other ideas?
Thanks,
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Knute Johnson