MBOX Breaker Trial

Apple Mail, for instance, has a 1 GB limit on the size of .mbox files it will let you import, so MBOX Breaker could split your existing mailbox into smaller files that can be successfully imported.

If you used a program like O2M to convert your Outlook PC mailbox into a Mac-friendly format, MBOX Breaker can split any oversized .MBOX files in order to maintain compatibility with programs like Apple Mail.
MBOX Breaker works with all types of .MBOX files, even those on Unix systems, and can help rescue you from having an "overstuffed" mailbox.