The best way to split large MBOX files into smaller files in SysTools MBOX Splitter tool. This software offers 5 options to split your MBOX files easily. You can easily divide your large MBOX files based on date, date-range, year, sender, and subject. It supports a minimum of 1MB and maximum of 20GB MBOX file sizes. Not just MBOX but you can also split MBS and MBX file formats smoothly.

Not just that, but this advanced MBOX file splitter offers a free demo version which splits 50 mail per MBOX file effortlessly. Try the free MBOX Splitter version of SysTools and check out for yourself before getting the full one. Moreover, this tool supports all versions of the Windows and Mac OS. Plus, it supports MBOX files from all supported email clients so no compatibility issues whatsoever.

Simply put, the SysTools MBOX Splitter utility is a top-notch software to divide large MBOX files by date, date range, year, sender, and subject. It is also recommended by countless technical experts as it is highly reliable and provides secure outputs for users.
Check out the utility here: https://www.systoolsgroup.com/mbox/splitter/