from handbook:
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Trusted Sender (e.g. Local/Outbound from the LAN)
Allow relay
Select this if you want the WinGate mail server to accept and send email for domains that are not local. Checking this box in the Trusted Senders section is not the security risk that checking it in the Untrusted section is because this only pertains to users on your LAN.
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I have this enabled but from some machines (not in the Windows domain but in the same IP subnet with the same domainname) I still have to pop before send. What exactly does trusted mean only boxes that are in the same windows domain?