I'm not sure how you'd be using Blat, but we always have an internal mail relay server that is either properly configured to authoritatively send email for our domain(s) or use a smart-host that is. Blat forwards email to that internal relay host. We'd never use Blat (or Outlook or any other mail tool for that matter) to try and send email directly.

Glenn
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Actually I am a Rocket Scientist! \:D