Where would information for this error be stored at?

Your software
My Mautic version is: 4.02
My PHP version is:
My Database type and version is:

Your problem
My problem is:

When I first created campaign email to test and it was successful. I deleted the campaign and tried again to test it again but none of the contact received the second email and all had this error notice in event record.

Where do I find more detail about this error? I looked at Mautic error log and it had no error recorded.

See screenshot below of the error flag

This looks like a re-occuring issue that many people are experiencing at the moment, firstly if you click on the toggle button on the left of the event you will be able to see the error.

You will also be able to see the error in Settings → System Info → Log.

This post probably covers your issues: mautic.ERROR: [MAIL ERROR] Expected response code 250 but got an empty response - #7 by mikew

@rcheesley I have now seen three posts today on the same/similar issue, maybe it is worth merging them all ?