Unable to Add Exchange 2019 Email Account (Ionos) to Outlook (New) on Windows 11

1. Contact the Ionos administrator
Ensure that Modern Authentication is enabled for the mailbox.
If it is not enabled, the administrator needs to enable it in the Exchange backend (path: Settings → Organization → Authentication).
2. Manually configure Outlook (key steps)
Open Outlook → File → Add Account → Enter mailbox and select Manual Configuration.
Select Exchange ActiveSync or EWS and fill in:
Incoming server: outlook.office365.com (port 993, SSL)
Outgoing server: smtp.office365.com (port 587, TLS)
3.Quick Verification
Visit the web version of Outlook Login directly to your mailbox to verify that your account is working.
If the web version is available, the problem probably lies in the client configuration.
Other considerations
4. Update Outlook to the latest version (check in Microsoft Store).
Check whether the system time and firewall block Outlook from accessing the network.