Email Queue unsent emails are suddenly sent

My Email Queue which were not sent from very long time, I am seeing suddenly yesterday there are triggered and sent.
No one has manually triggered also I am seeing Email sent at midnight 12 - 1 . I am suspecting is there any scheduler that retriggers unsent emails.

Hi,

Look at Scheduler Errors in Email Digest List and the scheduler can checked from command line by bench --site (sitename) scheduler status

The Email Queue List also has a Suspend/Resume Sending button to check

Got to know the root cause , In one of my schedule flush() was used which caused to retrigger unsent emails.
Now I am facing different issue. I have multiple sites in my setup. all other sites receiving emails. Only one of site emails remaining in queue. Not sent status. Any idea on this?

Click on the unsent email in the queue list and see if anything is shown to help explain.

Compare settings on site with the problem with the settings on the other sites. Are they all using the same domains etc?

Compare site_config.json for sites.

Email were sent today for all the sites, suddenly from 3 PM, emails not going for one of site. Now If I click on send now from UI still not sending.