Hi, Jero Cheng
It sounds like the issue you are experiencing is related to a security certificate error in Outlook.
This error message usually appears when the certificate used by the mail server is not trusted by the client computer
Here are the steps you may try.
1.Exit all Microsoft Office applications.
2.Start Registry Editor.
3.Locate and then select the following registry subkey:
HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\16.0\Outlook\HTTP
4.On the Edit menu, point to New, and then select DWORD Value.
5.Enter FailAllCertificateErrors, and then press Enter.
6.In the Details pane, press and hold (or right-click) FailAllCertificateErrors, and then select Modify.
7.In the Value data box, enter 1, and then select OK.
8.Exit Registry Editor.
More information can be found Outlook 2016 displays a prompt that lets you connect to an Exchange server if a certificate issue occurs - Microsoft Support