User Login From Domain to Only Username Exchange Server 2019

Domain Username to username login, default settings you need to use the Domain\Username structure however if you would like to login with Outlook Web App with a username please follow the below steps.

Domain username to username login

1- Login to the Exchange admin center

2- On your left side selects Servers and select the Virtual Directories tab and select OWA (Default Web Site) so, choose the Edit button.

3- Select Authentication and select User name only and click the Browse button.

4- Select the domain server and click on ok.

5- Now click on Save.

6- IIS restart warning, so clicks on ok.

7- Open command prompt and run the following command.

So, type iisreset

Or, open services and select the IIS Admin Service and click on Restart the service.

8- Now Restarting the IIS Admin Service.

9- Now log in to the Exchange admin center and select the Servers tab and select Virtual Directories and select ECP (Default Web Site) and click on the Edit button.

10- On your left side select authentication and select Use forms-based authentication and click on Save.

11- Now you can log in without a domain name.

If you need more details click here