choosing your archive: a technical showdown of email formats
for it administrators, developers, or anyone serious about long-term email archiving, the choice of file format is critical. it impacts data integrity, backup efficiency, and ease of access. while most users simply use the default format of their email client, understanding the differences between the three main open-source standards—mbox, eml, and maildir—can save you from major headaches down the road. let's break down the pros and cons of each.
1. mbox: the classic monolith
mbox is the original format. it stores all email messages—sometimes thousands of them—concatenated into a single, large text file. a special "from " line separates one message from the next.
- advantage: simplicity and portability. because it's just one file, moving or backing up an entire mailbox is as simple as copying that single file.
- disadvantage: high risk of corruption. this is the format's fatal flaw. a single error in the file, a corrupted "from " separator, or a system crash during a write operation can damage the entire archive, making all messages inaccessible.
- disadvantage: inefficiency. it struggles with simultaneous access (file locking issues), meaning two programs can't safely write to it at the same time. furthermore, incremental backups are a nightmare. if you receive one new 2kb email, your backup software has to re-copy the entire 10gb mbox file.
verdict: mbox is an outdated, inherently risky format. its continued use is largely due to legacy support from clients like thunderbird and apple mail.
2. eml: the individualist
the eml format takes the opposite approach. each email message is saved as its own separate file, complete with headers, body, and attachments, all adhering to the rfc 822 standard.
- advantage: robust and simple. corruption is isolated. if one .eml file gets damaged, you lose one email, not your entire archive of 10,000 messages. individual files are easy to manage, open in many different clients, and even view in a text editor.
- disadvantage: can be inefficient for large volumes. managing a folder with 50,000 individual .eml files can be slow for the operating system's file system. simple operations like listing files can take time.
verdict: eml is a huge step up from mbox in terms of data safety. it's excellent for exporting individual messages or smaller archives.
3. maildir: the modern architect
maildir was designed specifically to solve all the problems of mbox. it's not a file, but a directory structure. each mailbox is a folder, which contains three subfolders: tmp, new, and cur. every incoming email is saved as a unique, separate file, first in tmp, then moved to new, and finally to cur once it has been read.
- advantage: supreme robustness. there are no file locking issues; multiple programs can deliver mail simultaneously without conflict. corruption is almost a non-issue, as each message is an independent file.
- advantage: highly efficient. incremental backups are incredibly fast and efficient, as only new files (new emails) need to be copied. searching and deleting messages is also much faster as it involves simple file operations.
verdict: maildir is, from a technical standpoint, the superior format for email storage. it offers the best protection against data corruption and the highest efficiency for backups and server operations.
many users struggle with corrupt mbox files, a problem inherent to its 50-year-old design. maildir is the modern solution to all of mbox's shortcomings.
the bridge from old to new
if mbox is so fragile, why is it still so common? because it's the default export format for major services like google takeout. many users are stuck with this vulnerable format and need a safe way to handle it. if you are considering migrating your archive from the fragile mbox format to a more robust one like maildir, you need a reliable tool to inspect your data first.
use the mbox viewer chrome extension to safely audit and verify your mbox archive before any migration. our tool is built to handle the quirks and potential structural flaws of the mbox format, allowing you to access your data and ensure everything is intact before you move it to a safer home.