KoltrixExporting and backing up a mailbox: mbox, EML and PST compared
Your mail is only as safe as your last tested restore. What mbox, EML and PST files are, what each loses on the way out and how to back up a mailbox properly.
On this page(7 sections)
Most people think of their mailbox as permanent, because it has always been there. It is not. Accounts get suspended, a provider changes plan limits, a departing employee deletes a folder or a migration goes wrong. When that happens, the question is whether you have a copy and whether it works.
This guide explains the file formats you will meet when you export or back up mail, what each one keeps and loses, and a simple routine that gives you a backup you can actually restore. It pairs with team mailbox archive and retention habits, which covers how long to keep things.
Why back up a mailbox at all
- Provider risk. A suspended account or a billing problem can lock you out. Few providers promise you an easy export under pressure.
- Mistakes. A bulk delete or a bad filter can remove years of mail. Trash folders empty themselves.
- Departures. When someone leaves, their mailbox may be deleted by policy. See offboarding a teammate's shared email.
- Migrations. You need a safe copy before you move. A rollback plan starts with one.
- Legal and compliance needs. Some mail must be kept for a set period.
A provider's own redundancy protects against hardware failure. It does not protect you against deleting something or losing access.
The three formats you will meet
mbox: many messages in one file
An mbox file is a plain-text file that holds many messages one after another, each starting with a separator line. It is simple and old, and used widely.
- Opened by Thunderbird, Apple Mail (after import) and many conversion tools.
- Google Takeout exports Gmail as mbox, with labels written into a header.
- Easy to process with scripts, and easy to search with text tools.
- Large mailboxes become very large single files. A damaged separator can hide messages.
- Folder structure is usually one mbox per folder or label, and flags such as read or starred are not always preserved.
EML: one message per file
An EML file is a single message in the standard internet message format, including headers, body and attachments (encoded inside it).
- Opens in most mail clients and can be read in a text editor.
- Good for keeping one important message, a legal record or a sample for debugging.
- A mailbox of 100,000 messages becomes 100,000 files, which is slow on some file systems and awkward to move.
- Folder context is lost unless you keep the directory structure yourself.
PST: Outlook's container
A PST is Microsoft Outlook's proprietary data file. It holds mail, calendar, contacts and folder structure in one file.
- The usual choice for moving mail to or from Outlook for Windows.
- Microsoft 365 administrators can export mailboxes to PST through compliance tools.
- It keeps folders and many flags and it can include calendars and contacts.
- It is a closed format. Tools other than Outlook may need converters, and very large PST files can corrupt, so split big mailboxes.
- Treat a PST as a working file, not as an archive standard: also keep a copy in an open format.
A related file, MSG, holds a single Outlook message in Microsoft's format.
What gets lost in an export
Whichever format you choose, check what is missing before you rely on it.
| Item | Often kept | Often lost or changed |
|---|---|---|
| Message text and attachments | Yes | Rarely lost |
| Headers (From, Date, Message-ID) | Yes | Some tools rewrite or drop them |
| Folders | PST: yes. mbox: one file per folder | EML: only if you keep directories |
| Gmail labels | Takeout adds them as a header | A label with several folders becomes a duplicate when imported elsewhere |
| Read and starred flags | Some tools | Many conversions reset them |
| Threading | Preserved through headers | Some imports regroup by subject |
| Calendar, contacts | PST | mbox and EML do not carry them |
| Rules, filters and signatures | No | Always export these settings separately |
| Shared mailbox permissions | No | Record them in your own documentation |
If these details matter, test an export on a small mailbox and open the result in the destination.
Three ways to take a copy
- Use the provider's export. Google Takeout for Gmail, the Microsoft 365 admin and compliance export for PST, and the equivalent for other providers. This gives you official, complete data. It can take hours and may arrive as large archives.
- Sync over IMAP. A client or a command-line sync tool can copy a mailbox to local storage or to another server and can run repeatedly to keep a mirror. Protocol background is in IMAP, POP or Exchange.
- Use a backup service. A paid service can snapshot mailboxes daily and restore single items. Check where it stores data, who can access it and how restores work.
For a one-time move between providers, see moving from Google Workspace and migrating from Microsoft 365.
A backup routine that holds up
- Decide scope. Which mailboxes, including shared ones, and how far back.
- Pick a schedule. Monthly may be enough for a quiet mailbox, and daily for one that drives your business.
- Use the 3-2-1 idea. Three copies, on two kinds of storage, with one offsite.
- Encrypt it. A mailbox backup holds contracts, passwords sent in plain text and personal data. Encrypt the files and control access.
- Document it. Where backups live, who can restore them and how to do it.
- Test a restore. Once a quarter, restore one mailbox to a scratch account and read a few messages. Many backups fail only when they are needed.
- Keep an open-format copy. If you use PST, also keep mbox or EML in case the proprietary tool changes.
- Plan deletion. Old backups also fall under retention rules and data protection requests.
Privacy and legal points
A backup is a second copy of personal data. Include it in your privacy records, restrict access and delete it when the retention period ends. If a person asks for their data to be erased, decide how the request applies to backups, and write down your answer.
Key takeaways
- Back up mail because accounts, mistakes and migrations can take it away, not because servers fail.
- mbox holds many messages per file, EML holds one, and PST is Outlook's closed container with folders, calendar and contacts.
- Exports usually lose flags, rules and permissions, so test on a small mailbox and export settings separately.
- Follow 3-2-1, encrypt, document and test restores regularly.
- Treat backups as personal data under your retention and privacy rules.
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