I have a Proxy 2.0 and Exchange 5.5. There seem to be a few e-mail servers we cannot send mail to. We can receive mail from them, but their mail servers will not accept mail from us. We are not open relay and the error does not suggest an open relay problem. The error we get is: failure to communicate. There are two mail servers that we know of that we have this problem with. (I know one is an Linux box.) What do you suggest?
It sounds like Exchange might not be configured correctly to send messages through the Proxy server. Here are some KB articles on configuring Exchange for use in a Proxy server environment: Q181420, Q178532, Q181847, Q176947 and Q222134.
These articles include step-by-step instructions for configuring Exchange 5.5 for use with Proxy 2.0. Note that if it is misconfigured (and based on these symptoms, I really do think it is), there will be some restarting of Exchange services involved, and possibly some rebooting, as well. Just recently, there was a security hotfix released for Exchange 5.5; I strongly recommend installing that, too, which you can get here.
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