Archiving
Working with large MBOX files: open, split and back up
What to do when an archive is several gigabytes: why it fails to load, how to split it safely and how to store it for years.
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Email can be saved in several file formats, and the one you have decides which tools will open it. MBOX, EML and PST are the three you are most likely to meet. They solve different problems, so it helps to know how they compare.
Side-by-side comparison of formats MBox Viewer reads
| MBOX | EML | PST | |
|---|---|---|---|
| What it holds | Many messages in one file | One message per file | A whole mailbox: mail, contacts, calendar |
| File type | Plain text | Plain text | Binary database |
| Typical source | Gmail Takeout, Thunderbird, Apple Mail | Saving a single email, many mail apps | Classic Outlook for Windows |
| Folder structure | One file per folder | None (you organise the files) | Full folder tree inside one file |
| Readable without the original app | Yes, with a viewer | Yes, most mail apps | Needs Outlook or special tools |
MBOX in short for MBox Viewer users
MBOX joins messages end to end in a single text file. It is ideal for exports and long-term archives because it is simple, widely supported and readable decades later. Its downside is that large files are slow to scan and a single damaged section can disturb the rest.
EML in short for MBox Viewer users
EML is one email in one file, using the same internet message format that MBOX uses inside. You can attach an .eml file to a ticket, send it to a colleague or open it by double-clicking in most mail programs. In fact, each message in an MBOX file is essentially an EML file with a separator line in front of it.
That is why MBox Viewer can export one, several or all messages as separate .eml files. You get self-contained files without converting the whole archive.
PST in short and how it relates to MBox Viewer
PST is the personal storage format of classic Outlook for Windows. It is a structured binary file that holds mail, contacts, calendar items, tasks and notes together. Microsoft has published the specification, but in practice you still need Outlook or a dedicated library to read it.
Outlook does not import MBOX directly. That is the main reason people look for MBOX to PST conversion.
Converting formats before or after MBox Viewer
- MBOX to EML: export selected or all messages with MBox Viewer, or use Thunderbird with ImportExportTools NG to export a whole folder as EML files.
- EML to MBOX: import the EML files into a Thunderbird local folder. The folder itself is then an MBOX file.
- MBOX to PST: import into Thunderbird, connect an IMAP account in both Thunderbird and Outlook, and move the messages through the server. Paid converters also exist.
- PST to MBOX: open the PST in Outlook and move the mail through IMAP into Thunderbird, or use a conversion tool.
Before converting: ask whether you need to. If the goal is to read, search or keep a few messages, opening the MBOX file in a viewer avoids conversion entirely and leaves the original untouched.
Which format should MBox Viewer users keep?
- For a long-term personal archive: MBOX. It is open, text-based and supported almost everywhere.
- For sharing one message as evidence or reference: EML, or a PDF printout for people without a mail program.
- For continuing to work in Outlook: PST, since that is what Outlook expects.
Whatever you choose, keep the original export as well. Conversions can lose details such as header data, read status or unusual character sets.
