MacOS 13 Ventura Apple Mail Import mbox fails Your system has run out of application memory

Hey there Mike Richards1,

We'd like to help rule out some possible causes to this message showing when trying to import that mailbox, along with confirming more details to better understand what's happening:

1. The first thing we wanted to confirm is if you're using these steps to import the mailbox? Check out: Import or export mailboxes in Mail on Mac
2. From your screenshot, we see that only Finder and the Mail app are the only apps being used. Please open Activity Monitor, go to the 'Memory' tab, then organize it from what's using the most to least amount of memory in your Mac. Besides Mail and Finder, what's the next thing that's using the most memory, and how much is it using? You can open Activity Monitor by clicking on the magnifying glass near the top right of your screen, then searching for 'Activity Monitor'.
3. Sometimes unexpected behavior can be resolved with a restart. Check out the section ‘Restart your Mac' here: Log out, sleep, wake, restart, or shut down your Mac
4. You mentioned that macOS 13.1 is installed, but the latest version is macOS 13.2. You can verify this by clicking on the Apple logo at the top left corner of the screen, then on ‘About This Mac’. In case you needed it, this link shows how to first back up, then Update macOS on Mac.

Cheers!