Security certificate cannot be verified


I’ve configured a let’s Encrypt cert, got the green ok message and upon configuring Outlook, a “Security certificate cannot be verified” message appears:

The cert is apparently renewing correctly (Automated certificate renewal for has succeeded 2021-11-25 01:21)
DNS config for the domain is resolving directly to
Client clicks on Yes every time they start Outlook, and it continues coming back. It’s really bugging to them.
Anything I’m doing wrong?

So, just for clarifying, you’ve made a CNAME for pointing to and DA creates LE certificate for that without issue? (The root is irrelevant in this case.)

Are the Outlook and Windows version new/current, or older versions?
(Older versions will have issues with the root CA of current Lets Encrypt certs.)