Hello, @Michel Admiraal | Oranje IT,
Welcome to the Microsoft Q&A platform!
Based on your description, emails sent from "@domain.pt" and "@domain.ro" within the same Exchange Online tenant are being marked as spam with an SCL of 5, while emails from "@domain.NL" are not. Here are a few steps you can take to troubleshoot and potentially resolve this issue:
1.Domain Reputation: Use reputation-checking tools to see if the two domains have been blacklisted or have a poor reputation that could result in emails being marked as spam. Tools such as MXToolbox or SenderScore can provide insights into the reputation of your domains.
2.Check Anti-Spam Policies: Ensure that your anti-spam policies are correctly configured. You can do this through the Microsoft Defender portal or PowerShell. Make sure that the policies are not overly aggressive for specific domains.
3.Update Safe Senders List: Add the affected domains (@domain.pt and @domain.ro) to the Safe Senders list according to Configure junk email settings on Exchange Online mailboxes - Microsoft Defender for Office 365 | Microsoft Learn. This can help prevent emails from these domains from being marked as spam.
4.Review Advanced Spam Filter (ASF) Settings: ASF settings can sometimes be too aggressive and mark legitimate emails as spam. You can adjust these settings to be less strict for the affected domains with the help of ASF settings in EOP - Microsoft Defender for Office 365 | Microsoft Learn.
5.Check SPF, DKIM, and DMARC Records: Ensure that the SPF, DKIM, and DMARC records for the affected domains are correctly configured. Misconfigured records can lead to emails being marked as spam.
Please refer to Mail flow best practices for Exchange Online, Microsoft 365, and Office 365 (overview) | Microsoft Learn.
6.Submit False Positives: If legitimate emails are being marked as spam, you can submit these false positives to Microsoft for analysis according to Report spam, non-spam, phishing, suspicious emails and files to Microsoft - Microsoft Defender for Office 365 | Microsoft Learn. This helps improve the spam filtering algorithms.
Should you need more help on this, you can feel free to post back.
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Thank you for your support and understanding.
Best Wishes,
Alex Zhang