An mbox holds several messages in one file, each introduced by a From separator line at the start of a line. It is the format mail archives and account exports still use, and the one where parsers most often go wrong — because a body line that happens to begin with From looks exactly like a separator unless the reader is careful.

mbox/3-messages.mbox contains three plain-text messages separated by From lines, each with a fixed date and Message-ID. It is small enough to read in a text editor, so when a parser splits it into two messages or four you can see immediately where it went wrong.

Nothing comes from the machine that built it: dates and Message-IDs are derived from a seed rather than the clock, so the file is identical on every build and holds no personal data.

Use it to test mail importers, archive migration, search indexing and anything that has to iterate messages in a single file. Related format: EML for single messages.

Sample mbox file for testing mail importers — free, CC0

Related formats