EML, MSG or MBOX: choosing a format for long-term email storage
The export format decides what survives a change of mail client, a new laptop and a review five years later. We compare three everyday formats and explain why a PDF copy for reading belongs next to them.
Three formats, three scenarios
EML is one message per file.
- MSG is one message in Outlook format, with properties plain text does not carry.
- MBOX is a whole folder of messages in a single file.
- All three survive export from popular mail applications; they differ in how easy they are to work with once that application is gone.
EML: one message per file
An EML file is a single message per RFC 5322 with MIME parts: headers, body and attachments live in one file, and .eml has become the common extension.
- Thunderbird, Apple Mail, Outlook and dozens of tools open it; you can name it by date, file it into folders and forward it to a colleague.
- Losing one file does not touch the others — a valuable property for long-term storage.
MSG: convenient inside Outlook
MSG is Outlook's container based on the Compound File format: alongside the message text it stores flags, categories, statuses and other Microsoft properties.
- If your correspondence lives in Outlook and its internal attributes matter, MSG preserves the most.
- The flip side is that it is proprietary: mostly Microsoft tooling reads it, and another application may drop some of the fields.
MBOX: a whole folder in one file
MBOX puts all messages of a folder sequentially into one file — compact and simple.
- The drawback mirrors the benefit: while the file is intact everything inside is intact; once it is damaged you have to work through the whole document rather than one message.
- Different applications write separators and escaping slightly differently, so an MBOX from one client does not always open cleanly in another — worth testing before a migration.
Why a PDF for reading belongs next to them
The source format is good for processing and awkward for communication: opening an EML or MBOX needs an application, and you cannot paste a piece of a message into a chat or a memo.
- A PDF opens everywhere, prints well and looks the same for everyone — which is why an archive of message PDFs is what you send to an auditor, an expert or a new hire.
- Remember its role: the PDF here is a reading copy, not the source; any reprocessing starts from the original files.
An honest note on PDF/A and archival profiles
Some regulations call for PDF/A — the PDF profile designed for long-term archiving. eml-pdf.com offers PDF/A-2b output: every PDF is re-encoded with Ghostscript, fonts are embedded, interactive elements are dropped, and the state is recorded in the manifest.
- If re-encoding fails for a file, the archive keeps a regular PDF — the manifest shows it. Conformance is not checked by an independent validator, so it does not prove archival status; keep the original EML, MSG or MBOX files in the set — they remain the primary record.
A practical recommendation
Keep both: the source export in its original format, plus the ZIP with PDFs, the index and checksums.
- Once a year, check that both still open and that the checksum file matches current values.
- Do not let the only copy be something a single application can read: the combination of original, readable copy and manifest survives any change of mail system.