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How to handle Outlook Web login security warning when using a company’s custom SSL certificate?

Mai Đạt 0 Reputation points
2025-10-29T08:40:26.2333333+00:00

Hello community,

Our team recently configured Outlook on the web for business under a custom domain (example:Remove PII).

After setting up SSL/TLS on our mail server, some users are seeing a browser warning about “mixed content” or “untrusted certificate.”

Steps we took:

  1. Verified the SSL certificate using standard online tools.
  2. Checked DNS and MX records for proper propagation.
  3. Confirmed that HTTPS redirect and CSP headers are configured correctly.

Has anyone experienced similar behavior with Outlook Web access behind a custom domain?

Is there a recommended Microsoft best practice to ensure full SSL trust for Outlook on the web?

(For reference, here’s our public domain setup page — not promotional, just for configuration context: [Mod Removed] )

Thank you!

Outlook | Web | Outlook on the web for business | Security
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  1. Allison-H 5,040 Reputation points Microsoft External Staff Moderator
    2025-10-29T13:18:11.85+00:00

    Hi Mai Đạt

    Welcome to Microsoft Q&A forum 

    To better understand your issue, may I clarify a few details: 

    1.What is the exact error message shown in the browser? Could you please provide screenshot if possible? 

    2.Do your users sign in at Microsoft’s cloud site like https://outlook.office.com (Exchange Online), or do they access Outlook Web through your own company domain like https://mail.yourcompany.com (Exchange Server installed on your servers)? 

    3.Does this happen for all users or only some? 

    4.Does your SSL certificate include the full domain name and the complete intermediate chain? 

     

    The initial response may not fully address your needs, but it will help me better understand your situation and try to find the root causes. Thank you for your patience while we work through it  

    If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".   

    Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread. 

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