What is an MBOX file?

What MBOX is

MBOX is a plain-text format you can open in any editor. It is used for email and archiving.

The extension is .mbox, and the full name is Mailbox Archive. Both matter less than what the file can hold, which is what the rest of this page is about.

Where MBOX came from

It dates from 1975. The specification is RFC 4155.

A format that has been readable for that long is a format worth trusting with something you want back in ten years.

The specification is public

Anyone can implement it without asking permission or paying a licence, which is why it turns up in so many programs and why files written twenty years ago still open.

It can hold several pages in one file

A MBOX file is not limited to one page, which matters when converting to something that is: a target that holds one image produces one file per page, and the numbering it chooses is worth checking before you send it on.

What opens MBOX

Mozilla Thunderbird and Apple Mail read it, and so do most programs of the same kind.

If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.

Opening it in a browser

No browser reads it.

That is the single most common reason to convert it: not that the format is bad, but that the place you want to show the file cannot read it.

It is a delivery format

MBOX is meant to be handed over rather than worked in. Editing one is possible and rarely pleasant; the sane approach is to change the source and export again.

The facts, in one place

Formats worth considering instead of MBOX

• Email Message