No recption of external Email possible > returns 554 5.7.1 : Relay access denied

Error occurs after a fresh Debian installation and the Freedombox installed with the command line given in the documentation.

Problem Description
Running FreedomBox 24.21 and external Email can not be received by the Freedombox > mail is being returned to sender smtp; 554 5.7.1 Relay access denied

checked the DNS settings on the provider

questions >
where to check for corresponding log files what actually causes this behavior
what kind of steps can I undertake to narrow down the cause

Thank you for your help.