Login error to on-prem Exch 2019 from Outlook on internet. OWA OK.

Newly setup on-prem (no hybrid) Exch 2019 SU13 server. Internet logins from OWA work but from Outlook 2016 don’t, so far.

Local domain email logins from OWA & Outlook 2016 clients work.

https://testconnectivity.microsoft.com/tests/Ola/input works, its Ms Remote Connectivity Analyzer - Exch Server - Outlook Connectivity.

Outlook Error is: "Logon to ExchangeActiveSync email server (EAS): The username or password you entered isnt working. Please try typing them again". So it seems to be getting to the server (also looks ok in the firewall log).

OWA Logon name is: localdomainname\firstname.lastname

internet Outlook logon name: firstname.lastname@webdomainname.org

Outlook same error with logonname: firstname.lastname and firstname.lastname@mail.webdomainname.org

EAC - recipients - User logon name is: firstname.lastname@localdomainname.local. A popup says I can change that domain suffix (presumably to webdomainname.org) but it doesn’t let me; & the down arrow has no other options.

EAC - mailflow - accepted domains has 3 authoritative entries: webdomainname.org, mail.webdomainname.org, localdomain.local

btw email addresses are: username@webdomainname.orgThank you!