Hi, @Daniel Häberle
The answers above are all good explanations.
I'll just add a little more.
To improve the security of messages, Microsoft has been enhancing the accuracy of spam detection. Microsoft Exchange uses a combination of connection filtering, content filtering, outbound spam handling, and more to prevent spam.
You can learn more about anti-spam here Anti-spam protection - Microsoft Defender for Office 365 | Microsoft Learn
Your X-Forefront-Antispam-Report shows an SCL of 5, indicating that the email is considered spam. You can try this Use mail flow rules to the SCL in messages in Exchange Online | Microsoft Learn
Although both SpamAssassin and Barracuda believe that your message is not spam, Microsoft Exchange's anti-spam policy may mark it as spam based on other factors.
There are a few things you can do to reduce the likelihood of being identified as spam:
1.Don't just insert large images. Don't insert "IP-address only" links. Make it easy to unsubscribe or opt-out, or users will simply press the "spam" button and affect your reputation.
2.Choose a good SMTP server to avoid the IP address being often blacklisted by other providers.
3.Use SPF and DKIM.
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