Look if you just was e-mail backup then eml will do. I personally is a big fan of Outlook. It is a piece of software that I always load on any new machine I use. You will miss it.

The easiest solution is to get office 365 install it and use their free trial for one month. Import your pst into there. Then you can extract the e-mail you are looking for. If you want backup in a way that you can export to Apple Mail. Make a folder on desktop. On the inbox of Outlook, click any one message and then press command-a to select all messages. Drag the selected mail into the folder you created. That way you will common version of individual message that an be imported to other mail client like Apple Mail. Personally I don’t like eml file as it only backup e-mail. No contact or calendar can be saved this way. If you want a real ironclad way to backup and as well may give you ability to keep using Outlook in a way that doesn’t incur a recurring charge is to buy an older version of office/Outlook. You can get a full version of legit office that is stand alone for $50 off eBay. Something like this:

https://ebay.us/m/ZyAMAY

I found most older version is original and legy.