Prepare the split

Open Mailbox Utilities and choose Configure under Split Mailbox. When preparation finishes, choose a target size, one or more years, or one or more mailbox labels.

Split by target size

Choose a target in MB or GB. Messages stay in source order and each output approaches that size, but an individual message is never divided. A single message larger than the target becomes its own oversized part.

Split an MBOX archive by a target file size. View full size
MBox Office Split Mailbox window configured to split an archive into one gigabyte files

Split by year

Select the years to write. Every selected year becomes its own MBOX. Messages without a usable date are offered as Unknown Date. The summary previews file count, message copies, and approximate bytes.

Each selected year is written to its own MBOX file. View full size
MBox Office Split Mailbox window with six message years selected

Split by mailbox label

Select labels discovered in the archive headers. A message carrying several selected labels is written to every matching output, so the output can contain more message copies and bytes than the source. Review the summary before starting.

Choose mailbox labels and review duplicated message copies before splitting. View full size
MBox Office Split Mailbox window listing mailbox labels and projected output sizes