I’m trying to find out what client/application/program to use to read email/emails on my desktop/offline
I agree with the other when it comes to Postbox - a very nice desktop client and not too expensive (10 US dollar) if you do not have points to get it free. If you use gmail the integration works very well. I do also like the easy way to send Dropbox links instead of the files.
"Best" is a loaded word. Many people like Apple's own "Mail" app. Other's prefer Thunderbird or Postbox, and if you need compatibility with an Exchange server then MS Outlook is a distinct possibility.
have you tried Postbox (Working Offline)
Use Mail. That's the name of the app is "Mail." You need to find Mac Mail on your apps and also address book so you can put all of your email addresses in....