 What's the difference?
I have a basic Ubuntu/Zimbra box set up for test purposes. *None of my MX records have been directed to this new box yet as I'm not ready for it to go live* I'm just testing outbound. If I can send to Verizon and it goes through perfectly, but fails sending to Yahoo, what is Yahoo doing different from Verizon? Why would one go through and the other not? Does yahoo do some sort of verification of the source? I am using Verizon as a service provider. Perhaps they just allow any mail coming from their own networks? | 
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