Free Online MBOX Viewer
Open and read any .mbox archive in your browser. Every message listed, with headers and attachments intact.
Built by OnlinePCApps, mailbox tooling since 2013
When Nothing Opens Your MBOX File
Three situations account for almost every visit to this page. All three end the same way.
You Are Leaving Thunderbird
Each folder in a Thunderbird profile is an mbox file with no extension. You want to see inside one before committing to a move.
A Google Takeout Export
Takeout hands back one enormous archive covering the whole account and offers no way to look inside it.
A Retired Mail Server
Postfix and Dovecot wrote mbox spools for years. The server is long gone and the spool files are all that survived.
Open an MBOX File Online in 3 Steps
Pick the Archive
Drag the .mbox in or browse to it. Your browser lifts the archive from local storage, so there is no upload bar to sit through.
Browse the Messages
Every message is listed by sender, subject and date. Open one for its body, its header chain or its attachments.
Take What You Need
Save an attachment, lift one message out as EML or send it to PDF. Shut the tab and no trace stays behind.
Which Programs Write MBOX Files
The extension changes and sometimes there is none at all. What sits inside is the same layout every time.
Mozilla Thunderbird
Every folder in a profile is an mbox file with no extension at all, sitting under Mail or ImapMail.
Google Takeout
Choosing Mail produces one archive holding the entire account, spam and trash included.
Apple Mail
Exports a .mbox package, which is a folder wrapping the real file rather than the file itself.
Unix Mail Servers
Postfix, Sendmail and Dovecot have written to mbox spools for decades. The spools outlive the servers.
This MBOX Viewer vs Upload Services
Most online MBOX viewers post your archive to a server, hold it in a queue, then hand back a result. Here is the bill for that.
| Capability | This page OnlinePCApps Runs inside your browser | Typical upload-based service Hands your archive to a server |
|---|---|---|
| Your archive is handed to a server | Never | Yes, on every open |
| Size tier behind a paywall | None | Commonly 25 MB |
| Sign-up required | No | Usually above the free tier |
| Time to the first message | Under a second | Upload time plus a queue, often minutes |
| Cost per archive | $0 | A subscription above the free tier |
| Result is deleted | Never, it stays local | Usually purged after 24 hours |
| Your archive leaves the device | Never | No |
| Deletion promise required | Not applicable | Unverifiable by you |
This describes how server-side viewers generally operate rather than any one named product.
Your MBOX File Never Reaches Our Servers
Every upload-based viewer has to offer the same reassurance: your archive will be deleted afterwards. That reassurance is worth exactly as much as the company behind it.
This page offers nothing of the kind, because there is nothing to offer. No endpoint receives the archive and no server-side component exists for this app. Open your browser developer tools, load a file and watch the network panel stay still. The archive is opened read only as well, so headers, dates and message order survive untouched.
→ external requests ....... 0
→ your archive ....... 0 requests
What an MBOX File Actually Contains
An MBOX file is many messages written one after another into a single plain text file. Understanding how they are separated explains most of what an MBOX reader has to do.
| Part | Header | What it holds |
|---|---|---|
| Separator | From | A line starting with the word From and a space marks where one message ends and the next begins. It is the only thing dividing them. |
| Escaping | >From | A body line that genuinely starts that way gets a greater-than sign added so it is not mistaken for a boundary. Readers strip it again on display. |
| Envelope | From, To, Cc | Ordinary address headers, repeated once per message inside the same file |
| Subject | Subject | Message title, encoded whenever it carries characters outside plain ASCII |
| Timestamp | Date | Send time with a timezone offset, one per message |
| Routing | Received | The chain of servers that handled the message, newest first |
| Structure | Content-Type | Declares the body as text, HTML or multipart and names the divider between those parts |
| Encoding | Content-Transfer-Encoding | How non-text content is folded in so a plain text file can carry it, whether that is base64 or quoted-printable |
| Attachments | Content-Disposition | Identifies a part as an attachment and holds its original name. A 2 MB photo swells to nearly 3 MB once encoded |
| Gmail labels | X-Gmail-Labels | Added by Google Takeout only. Carries the labels that were on the message when it was exported |
| Status flags | Status, X-Status | Read, replied and flagged state, written by whichever client owned the file |
Standards behind the format
Mbox is unusual in having no single governing document. The media type is registered, but the registration itself notes the layout grew as a convention rather than a specification, which is why two readers can disagree about the same file. The messages inside it, however, are ordinary internet mail and follow published IETF documents exactly.
Newer archives may also carry RFC 6532 headers, which place UTF-8 straight into the field instead of encoding it. Both are read here.
Other Ways to Open an MBOX File
This is not the only route into an mbox archive. Here is what else works and what each option costs you.
Import Into Thunderbird
Open in Apple Mail
Read It in a Terminal
Rename It to .txt
Rename It to .txt
Use archive.mbox with archive.txt and open it in a text editor. The separator lines are visible straight away, which confirms you really do have an mbox file rather than something misnamed.
Split It With a Script
Use csplit with formail on Linux or macOS will cut an archive into individual messages at each separator line, which is enough to isolate one message for a colleague.
When an MBOX Archive Outgrows the Browser
A tab can only do so much. The desktop build exists for the jobs waiting on the other side of that line.
Why There Is a Desktop Build at All
A browser holds the whole archive in device memory while it works through it. For a folder export that is fine. For a Takeout file covering fifteen years of mail it is not. A phone gives up a long way before a laptop does.
The desktop build reads in pieces from disk instead, so archive size stops being something you have to plan around.
No connection needed once installed. It works on air gapped estates and inside networks that drop outbound traffic.
Archives in the tens of gigabytes open without the tab dying, because nothing is ever held whole in memory.
Aim it at a directory of archives and step between them like folders instead of reopening one file after another.
Several archives queued together without the memory ceiling a tab runs into.
Locate one message across every archive on disk by who sent it, when it arrived or what the body says.
Turn a whole archive into PDF in a single pass, headers and all, which is what disclosure bundles ask for.
Questions About Reading MBOX Files
Other Email & File Apps
Every one of these runs the same way: in the browser, at no cost.
Who Built This MBOX Reader
That MBOX Archive You Cannot Open
Drop it in and start reading. Nothing installed, nothing signed up for and nothing sent anywhere.