Lately I get constant Pop Up messages when using Outlook (in Office 365 programme)

The message is a Kaspersky Total Security message reading
"Cannot guarantee authenticity of the domain to which encrypted connection is established" and it refers to it coming from "Microsoft Outlook " application.

I can stop this by entering the URL into Kaspersky and then Kaspersky will ignore this problem but I don't know if this message is something to be concerned about so I don't want to simply ignore the message.