Hi @Markus_1981
Welcome to our forum!
What is your account? Outlook.com or something else?
Does the issue occur recently on Outlook Desktop Client? Do you have any specific operation before the issue occurred?
According to your description, the issue of your concern is that your sent emails marked as spam.
First, please be kindly to have a check for your domain Office 365 Anti-Spam IP Delist Portal.
Also, it is suggested please kindly go to Domain to set up page from your Microsoft Admin center and check if TXT records are correctly updated and have no issue. If records do not update correctly or any error, kindly contact your local domain provider and update the record.
- Micosoft365 portal > Admin center > Settings > Domain > select your affected domain and check DNS records.
If all above works fine, could you please ask the recipients to add your domain into the safe list( as safe sender) to see if it helps.
If not, you may want to refer to the article Remove blocked users from the Restricted entities page - Microsoft Defender for Office 365 | Microsoft Learn for more details.
Hope it helps!
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