ValentinaStudio is a free application available in the AppStore or from the Valentina homepage. Mail Archiver can't export data directly to CSV but ValentinaStudio can.

If you use the version from the AppStore you need to tell ValentinaStudio which folders it can read and write in the Preferences.

In ValentinaStudio click on the Open Database button:
ValentinaStudio: open database

Select an archive and the database will be opened. The email data is in the Message table. Click to open the table:

Database open in Valentina Studio

The Message table has been selected:

Table message selected

Do a context menu click on the Message table to get the context menu:

Context menu

Click on Export to CSV... You will now see the Export screen:Export screen

We need to select a file and a couple of parameters. Click on the Select button and then enter a file name and a location for your CSV file:

Export parameters

There are a couple of field delimiters available. Select Semicolon or Tab. Excel or Numbers should be able to handle all line delimiters. Change as needed. Encoding can stay as UTF8.

Now comes the most important part: which fields to you want to export? The database fields don't have cryptic names. OriginatorFrom is From and so on. Only the three fields for the message body (MessageBody, MessageBodyHtml, MessageRawData) aren't suitable for Excel. Therefore, I have excluded the 3 fields in the screenshot above.

Click on Continue and the data will be exported:
Data exported

Click on Done and we are finished in ValentinaStudio. The file has been exported to the desktop:

Exported file on desktop

Open Excel and then open the CSV file. Excel will recognise the file format and will show you the Text Import Wizard:
Text Import WizardSelect the same delimiter that you used in ValentinaStudio. I selected Semicolon as Delimiter. In the lower area of the screen you should now see the fields from the Message table. This is the critical part. If you don't see the fields separated or you see some sort of mess then check the field delimiter, the line delimiter in both ValentinaStudio and Excel.

Click on the Finish button and we now have the data in Excel:

Data has been loaded into Excel

That's it!