Have you checked to make sure that your domain has the correct reverse DNS entry?

I have to admit that I'm confused. The first half of your post indicates that emails you send get bounced back to you. The second half talks about how you force the Astaro to deliver emails that would be blocked by the spam filter. Rather than disable the SMTP Proxy, why not whitelist the improperly-configured domain for RDNS/HELO?

Cheers- Bob

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