Personally I think just flag it and ignore it.

The "threat" is identified as a javascript threat. Thunderbird does not run javascript on email bodies, like having ammunition in your home, it is not dangerous unless it is loaded in something that can make it go bang. Like Microsoft Outlook, which will execute the javascript. In Thunderbird all it does is consume a few bytes of space. Completely inert.

The file you have identified is a plain text file, It is an MBOX file (RD variant) with a small Mozilla twist in the formatting of the From line. MBOX is the default mail storage file format for Thunderbird. There is nothing to extract from it other than text so of course 7-zip is not going to extract anything, nor is Esets deep scan.

You can open the file and read it in windows notepad if you want to see what is in it. What you will see is the raw text that makes up the emails source in the folder one email following he other. essentially a stack of EML files. https://www.loc.gov/preservation/digital/formats/fdd/fdd000385.shtml

I have no idea what rubbish the Eset folk are selling you in their support, I did used the product until a couple of years ago and around 10 years ago it was very good. A couple of years ago I figured the money I was giving them for a load of alarmist warnings about javascript marketing and tracking code on web pages was a poor exchange. So now I use defender.

Generally antivirus scanners actually decode the mime portions of emails so they can examine the HTML body and attachments. Apparently Eset do not do this. Personally I wonder why they do this scanning at all in text files. For years I have been recommending to people that they exclude the Thunderbird profile from all scanning activity other than periodic scans, because A/V usually causes more issues with email than it solves ad this is a perfect example.

Eset have consumed your time, and now mine discussing what is in essence a false positive as it can not actually take any action on your computer unless you save the email as HTML and open it in a web browser. I am sure some people do that, but the A/V product will get two more bites of the apple, once when the HTML file is written/saved, and again when it is loaded in the web browser. They fed you full of rubbish about deep scans for a text file. What they think they can extract from a text file I have no idea at all.

Thank you Matt for taking the time for your explanation and reply!

I'm not referring to the .MBOX file. The two .MBOX files is from Google Takeout's data export feature. However, that is also a similar case, where I'm not being able to view the contents of the 14 GB .MBOX file, as well.

I checked the file in Notepad as you suggested, and see only text. The original emails have some attachments which don't reflect in the Notepad file.

Is it the attachments which ESET is catching these security detections? I mean, why would it detect the security detections only in some of the emails and not all? That means, there's something in some of the detected emails, right?

I wish there was some way for me to view the contents of the specific Thunderbird file which I have highlighted in pink in the screenshot.

Q: Would you know a way, to open the 'PG Campus' folder in Thunderbird's app interface and scan the full folder of 58 emails, via the ESET plug-in, so that I can see if it pin-points which particular email(s) may have security detections?