We've had a couple of users recently where their Outlook client just refuses to connect properly to our on-prem Exchange Server (we're on 2016/2019 mix). They see "Trying to connect..." or "Disconnected" in the status bar, get constant password prompts that don't seem to work even with the right credentials, and obviously, no new mail syncs.
Checked the basics – network connectivity is fine, user account isn't locked, password hasn't expired, other web services are reachable. It's incredibly frustrating when it just stops working for seemingly no reason.
After digging around for a while, I found a really comprehensive checklist of things to investigate. It covered more than just the usual network stuff and went into Autodiscover configuration, potential certificate problems, how to test connectivity properly, and even checking for corrupted Outlook profiles.
If you're running into similar headaches, this resource might save you some time. It outlines a bunch of potential causes and troubleshooting steps:
Troubleshooting steps for when Outlook isn't connecting to Exchange Server