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Error 553 or error 503 -"not in list of RCP hosts", "relaying denied" - or "Server Error: 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)"
FIRST, make sure you have "My Outgoing Server Requires Authentication" checked in the "Servers" tab of your email account setup (Outlook, Outlook Express). This error could also relate to ISP send restrictions
You may also find that if you run "receive" email and then send again the message will go right through. Why? Because as a defense against spammers our server blocks email sending if email receive has not been accessed within the last 30 minutes. Run Send Receive a second time and the message may go out without any problem.
We use a recognized spam blocking filter service and a separate Spam Assassin function which YOU control in your server control panel.