[Koha] Where are SMTP errors logged?

I defined an SMTP server for one of my koha libraries, but it is failing to send messages. Another library (on the same server) is working OK with the same SMTP server settings, as far as I can see. I have set the Debug mode to "enabled" on the SMTP server settings. Can you tell me where the debug output goes? We are using koha 22.11 Thanks! Chris Brown