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Outlook 2016 - security certificate issue when opening outlook

mmj1 asked
I have Office365 locally installed on a PC with the email account setup as IMAP.  All was working ok but now have a few issues:
1) When opening Outlook frequently (not always) getting a send/receive error - usually a reboot will correct
2) Yesterday - started getting the following message:  "Internet Security Warning" - The server you are connected to is using a security certificate that cannot be verified..... do you want to continue using this server.

I have two different email accounts setup in Outlook (one goes through Yahoo servers and the other through Total Server Solutions (previously Hands On Webhosting) - both accounts are setup as IMAP.

Any help is appreciated!  Thank you.
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Sr. Consultant/Managing Partner
Secure IMAP for both accounts? Are both accounts showing the security certificate error?

Has anything changed with your hosts file...that question is a big stretch but you never know.

Another big stretch...Windows update. When exactly did the error(s) appear? Can you trace that to a Windows update?

Have you looked at the details about the certificate that is providing the error? There should be a details button or link to get to the certificate? Look through the details. Often this error means something DNS doesn't match or the certificate that is likely imported into your trusted certificates store (one of them) is expired.

Please let us know...

David FavorFractional CTO
Distinguished Expert 2019

My guess is this relates to a broken load balancer or proxy.

Better to test + know, than guess.

Provide the hostname of the IMAP server you're connecting to for testing.

If you must do this yourself, what you're looking for is if the IMAP instance has a fixed IP or an IP pool.

What you describe sounds like an IP pool, where one or more IPs have some config which is incorrectly serving your cert.


Thank you both for your input.  After further review, we think this issue with the "Internet Security Warning" is due to a site we are navigating to that needs to renew the SSL certificate - that is being worked on so I'll know once they renew it if that was the problem.   I am going to close this question and award you both points for your assistance.  If the cert issue is not resolved by the renewing of the cert I may need to log another question.  Thanks again.