Binary index
A compact index file that Mbox Viewer writes alongside an MBOX archive after the first parse, storing message byte offsets and metadata to enable near-instant reopens without re-scanning the entire file.
After Mbox Viewer opens and streams through an MBOX file for the first time, it writes a small binary index file to the same location. This index records the byte offset and length of each message in the MBOX, along with key header metadata such as date, sender, and subject. On all subsequent opens, the app loads the index rather than re-scanning the file, reducing open time from minutes (for very large archives) to under a second.
To ensure the index stays valid, Mbox Viewer stores a SHA-256 hash of the MBOX file (or a portion of it sufficient to detect changes) in the index. When the app opens an MBOX file, it verifies that the hash matches before trusting the index. If the MBOX has been modified — for example, because a new export replaced the old file — the index is discarded and the file is re-parsed.
The binary index is a performance optimization that is entirely transparent to the user. It never modifies the MBOX file itself; it only creates a companion file. This is consistent with Mbox Viewer's read-only design: your email archive is never altered, only read.
Related terms
A parsing technique that reads a file incrementally in small chunks rather than loading the entire file into memory at once, enabling tools to open and index very large MBOX files — tens or hundreds of gigabytes — with low memory usage.
A plain-text file format that stores multiple email messages concatenated together, each beginning with a "From " separator line. It is the format Google Takeout produces when you export your Gmail archive.