Google Takeout
Google's official service for exporting your personal data, including Gmail. For email, it produces one or more MBOX files containing all your messages and their Gmail labels.
Google Takeout (takeout.google.com) lets any Google account holder download a copy of their data from Google's services — Gmail, Google Drive, Photos, Contacts, and many others. For Gmail, Takeout exports messages as MBOX files, preserving the full message content, headers, and label information. Depending on mailbox size, Google may split the export into multiple MBOX files or offer different size limits per file.
The export process typically takes from a few minutes to several hours or even days for very large mailboxes. Google sends an email notification when the archive is ready to download. The resulting MBOX files are a complete, offline copy of your Gmail that you own independently of Google's servers.
Once downloaded, you can open the MBOX files in any compatible application. Mbox Viewer is designed specifically for this use case: it opens the MBOX files from Google Takeout without any import step, streams through files of any size, and reconstructs the Gmail label hierarchy in the sidebar so you can navigate your archive just as you would in Gmail.
Related terms
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.
Gmail's tagging system that assigns one or more labels to each message, serving the role that folders play in traditional email clients. A single message can carry multiple labels simultaneously.