Logfiles for CMS.EmailEngine.ThreadSender?

There are no log files for this. What kind of logs are you looking for? If there is some exception, it should be logged in the Event log - but sometimes the thread or application may get killed at very low level so it will not get into the Kentico Event log. You may want to check also the Windows Event viewer or, if running e.g. on Azure, check the Azure logs and insights.

Thanks Juraj. I'm having an issue with the CMS.EmailEngine.ThreadSender process not ending after all the emails in the queue have been sent. No new emails added to the queue will send until I kill that process and then run the Send Emails scheduled task. I'm running in Azure AppServices, so I don't have access to the Event Viewer, and I haven't seen anything in AppInsights.

I would take the backups from Azure, restore it locally on a testing machine and try reproducing the issue there. Maybe it is environment related - something kills the sender thread abnormally and Kentico does not know about it and thinks the thread is still running.