One or a few users are not able to connect to their mailbox using the newest Outlook version there is? Well the issue might be TLS 1.2 not being active on the user's side.

Just set this registry key and you should be good to go:

HKCU\Software\Policies\Microsoft\Windows\CurrentVersion\Internet Settings

DWORD SecureProtocols 00000A80

HEX A80 or decimal 2688 enables TLS 1.0, 1.1 and 1.2, while decimal 640 only enables TLS 1.0 and 1.1.

References:

https://www.anoopcnair.com/disable-enable-tls-1-0-1-1-internet-explorer-ie/