Hi @mwc2767,
Certificate errors warn users that the certificate on the server is expired. The certificate isn’t “installed” on the client, but is presented by the server to the client when the client initiates a connection. (A somewhat frustrating recent forum thread in which various community members attempt to explain this issue is found at Let’s Encrypt got on my iphone without my permission in case you’re unclear on the overall concept that the certificate isn’t “installed” on the client device but rather provided by the server. However, if you accept this there’s no benefit to reading that other thread.)
As @JuergenAuer says, we can’t likely diagnose this problem without knowing the real domain name.
The most likely interpretation is that the clients are configured to use some kind of TLS service such as SMTPS, IMAPS, or similar, pointed at a server which is using an expired Let’s Encrypt certificate which needs to be renewed.
If you don’t want to share the domain name but want us to speculate on this, although it will be much more difficult and annoying, please at least provide screenshots of the actual Outlook error as well as screenshots of the Outlook configuration on an affected machine.