Outlook client prompt: Cannot Verify Security Certificate warning

Question

Hi,
I have a group of users using O365 mail (F1 license).
They are using Office 2016 suite, connecting to O365 mail via POP3.
They all scatter in different locations. They all have their own internet access.
Suddenly, many of them prompt this error message.
They have been using O365 mail for couple of years, everything was working fine.
Please see pictures, hope you can guess and understand the content.
Can someone help?

I have 3 more pictures would like to share.

Answer 1

Hi @WONG Steven ,

Can outlook still work fine if you click yes to continue using this server?

I noticed that you mentioned that many users have the same issue in your organization. Does this issue happen to all the users who connect via POP3, or just part of them? Please check if there is any user who connect via POP3 can work fine without any issues. If there indeed some, maybe we could try comparing the difference between them.

As you mentioned, since this issue occur to multiple users at the same time, I'm afraid the issue might not be related to your Outlook desktop client. I did some search about this Security Warning and found that this warning indicates the server name does not match the name in your account settings, or in the case of Exchange server, that the address in the autodiscover file does not match the address the server is using.

In order to conduct further researches on your issue, it is suggested for you to contact your administrator to confirm if there is any changes which might cause issues like this applied among your organization.

If the issue still continues, I found some similar thread of your issue, please try the workaround in these articles and see if they could help:
deactivate warning unable to verify certificate Outlook 2013
Security Certificate Warning in Microsoft Outlook
Cannot verify Security Certificate warning
(Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)

Any update, please feel free to post back.

---

If an Answer is helpful, please click "Accept Answer" and upvote it.
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.

Answer 2

Hi Jeff,
Outlook still work find if users click yes to continue.
I have 20 users connecting via POP3, I cross-checked with them, they all received this message randomly over the last few weeks. All of them had this problem.

I am the IT administrator of this organization, nothing was changed recently.

I review the links you provided:

• deactivate warning unable to verify certificate Outlook 2013
• Security Certificate Warning in Microsoft Outlook
• Cannot verify Security Certificate warning
  All of them were related to certification issue. It doesn't seem related to our case.

Our O365 mail service is provided by Microsoft. This group of user use POP3 and O365 mail (F1 license) for couple of years. This issue started happening around 3 weeks ago. Users are all located in different site, not likely a local infrastructure issue. If the problem is on Server side, Microsoft have to investigate the problem.
Look forward to hearing your reply.
Regards,
Steven

Answer 3

Hi @WONG Steven ,

Thanks for your reply.

What exactly will shows if you click the "View Certificate" button. Please try to check whether the information there is consistent with your server name.

Outlook still work find if users click yes to continue.

Will this prompt happen again after you clicked yes? If so, please try paying attention to when this pop-up window will appear again(after specific operations or some other things). From the perspective of Outlook client, it is suggested to select one of these problematic device to test, try creating and using a new Outlook profile via Control Panel > Mail > Show profile and see if the issue will appear again.

I did similar tests about your issue, add Office 365 email account as POP account to Outlook desktop client, run for quite a while but could not get the same prompt as you. To confirm if the issue is related to the connect between Outlook client and mail server, please try to run the Microsoft Remote Connectivity Analyzer tool to see if there is any errors between your device to your mail server.

If you have any update about this issue, please feel free to post back.

---

If an Answer is helpful, please click "Accept Answer" and upvote it.
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.