I had a user report this result when sending to known valid and functioning xmail. The message doesn't like like one from xmail to me - but I don't know what to make of it.
<Failure Reason> Message rejected by server filters </Failure Reason> We don't run any filters of this type. - To unsubscribe from this list: send the line "unsubscribe xmail" in the body of a message to [EMAIL PROTECTED] For general help: send the line "help" in the body of a message to [EMAIL PROTECTED]