[TonyGayle]

Outlook Mac mails are different from Apple Mail.
to sync the file format differences, you will need a third party tool that can convert the Outlook Mac mails to apple Mail format. I have used OLM Converter Pro by Gladwev in the past.
you can convert upto 10 mails per folder for free.

[timharison]

Try Mac PST to MBOX Converter to move your data from Outlook PST to Apple Mail. This tool Migrate the data simply and quickly in Mac system without damaging any file. Then you can easily import that converted MBOX file in Apple mail email client and there is no error you will face. Visit here for more details:- https://www.toolscrunch.com/mac-pst-to-mbox-converter.html

[emerywill]

PST to Apple Mail Converter Tool is the faultless answer. It lets to export multiple PST files to Apple mail at once without losing data. This application provides the complete preview of emails before migrating them to Apple mail format. PST to Apple mail Conversion is useful to rename the migrated file. This utility is well-matched with all the versions of Windows & MS Outlook. For more details please visit:- http://www.pstfile.org/applemail/