Open Mail.app without an account for read only access to a .mbox file

Mail.app has been ens***ified and so you can no longer use it without providing a username and password to /some/ email account. Even if you don't want to use that email account and you just want to browse mail inside your .mbox that you exported from Google Takeout or wherever.

So you will need to use another software. This is heavy, so take notes.

Assuming you don't want to access your mail using Terminal. And assuming you want open source software. Your own option (as of 2024-11-12) is Thunderbird.

1. Buy and install Little Snitch (because TB is offensive, it has 5–10 hidden telemetry actions when you first open)
2. Install TB
3. Cancel out of all the setup steps
4. Setup a new Feeds account [sic]
5. In settings change your Feed account's local directory to the folder containing your .mbox file
6. Restart Thundbird
7. Switch to the Mail icon on left
8. Click onto the .mbox "folder"
9. You should quickly (<10min) see your mailbox message account if it is less than 50k messages, e.g. 10 GB, using a 2022 MacBook Pro or similar
10. Within 30 minutes indexing is complete and you should have full access to the mbox.