Below are the 2 processes that will help you avoid the certificate error. You can do either or both on the system having this issue.
Process-1
• Click on windows button and write Allow an Apps through windows firewall or you can search this in control panel also.
• After that, click on change settings(user need to the admin of that system)
• search for Microsoft Outlook and check all the available options (Domain, Private, Public)
• restart the system.
Process-2
• Open Registry Editor
• search for HKEY_CURRENT_USER \ Software \ Microsoft \ Office \ 16.0 \ Outlook \ HTTP
• Once inside HTTP, right click on the white screen and click on new.
• Create a 32-bit Dword file
• on asking the name, please enter- FailAllCertificateErrors and assign it value 1
• restart the system
Please let me know if you further need my assistance.