Header Anomaly - two different mailserver - same domain

There are a number of ways to handle this. Easiest and the way I would recommend is create a separate Email Security policy where the sender is newletter@abc123.com in the External tab and turn off Header Anomaly for that policy. The only time that policy will match is when the sender is newsletter@abc123.com.

Thanks for your response and idea.
As soon that "exclusion" gets known then somone else can exploit it, right?

That is a risk but if all you are relying on is Header Anomaly for email protection then you are missing the value of the product. You also haven't stated whether your newsletter provider is a permitted sender in your SPF record which would combat the spoof attempt or whether you have done due diligence and have a DMARC record properly setup. You did mention DKIM but as you stated it doesn't solve for internal but I would also assume you've provided a DKIM key to the vendor sending on your behalf. So email security like other security technologies should not be dependent on 1 single engine but a multiple engines that make scoring decisions based on multiple factors to determine the authenticity of a message. 

Okay true thanks for your comment.
I would like to configure it like you mention above. I also tried it already but the mail always appears in the quarantine now.
Do you have any idea why so?

I got 2 policies now.

On top is the new one which I configured like you showed: I added the newsletter adresse and I put header anomaly tagged as banner

in the 2nd base policy I switched it from tagged as banner to move into quarantine.

Both rules are enforced.

So I thought its top down now.

makes sure you are entering envelope sender in the External list.

It's always Top Down First rule it hits gets applied. In your example newsletter@abc123.com is likely the From address not the envelope sender address.

It's always Top Down First rule it hits gets applied. In your example newsletter@abc123.com is likely the From address not the envelope sender address.

the envelope sender address for amazon Simple Email Service does always seems to switch randomly.
any ideas what I can do here?

Always lands in the quarantine