There are many software which are available in the market to convert emails in windows. But there are only limited ones which can do in both windows and macos.
I was doing a research for a tool which has the capability to do both and i found Kernel Email Converter.
This tool worked well for me as it was able to do conversion in both Windows and MacOS.
Free trial is also available of the tool if anyone needs to take a demo of the software.
If you need an email converter that works on both Windows and macOS, I’d look for one that supports the mailbox format you actually have instead of installing separate tools for each operating system.
WholeClear Email Conversion Tool is one option worth considering. It supports Windows 10/11 and macOS 12 or later, including Intel and Apple Silicon Macs. It covers common email conversions such as MBOX to PST, EML to PST, OLM to PST, PST to MBOX, NSF to PST and OST to PST, so it can be useful when moving mail between different email clients.
What I like about this type of setup is that you can preview the source mailbox before conversion and check the data you are about to move. It is also designed to retain attachments and email details such as sender, recipient, CC/BCC, subject and date/time.
If you only need one conversion, you can select that particular converter rather than buying the complete package. There is also a trial version, so I’d test a few emails and attachments first, especially before processing a large mailbox.