What is the BOX file format?
A .box file is an email mailbox container used by several older and niche email clients to store messages, folders, and related metadata in a single file. Unlike the standardized mbox format or Microsoft's .pst, the .box extension is claimed by multiple unrelated applications with incompatible internal layouts.
The most common modern user is Foxmail, a popular email client (particularly in Chinese-speaking markets), which stores each mailbox folder as a separate .box file alongside a companion index. Foxmail's .box files are partially binary - messages are stored sequentially in a proprietary record structure rather than the plain-text From separator used in mbox.
The discontinued Calypso Email client (circa 1994) used .box files closer to a text-based mbox layout. Microsoft Systems Management Server (SMS/SCCM) also uses .box files as binary package transport containers - an entirely unrelated use sharing only the extension name.
Because there is no single specification, .box files from one application are generally unreadable by another. Mozilla Thunderbird can import Foxmail .box files via its built-in import wizard. Direct conversion to .eml files (individual RFC 2822 message files) is the most portable migration path.
The format is considered legacy; modern email clients prefer standardized containers such as MBOX or proprietary formats like PST, which offer better tooling and interoperability.
Security & safety
RISK: LOWA .box mailbox file is passive data (email messages). The emails inside may contain phishing/malware links but the container itself is not executable. BOX files from SMS/SCCM distribution points are admin-managed packages - not end-user-facing.
Format details
in a nutshell- Microsoft Systems Management Server (SMS/SCCM) Package - SMS/SCCM uses .box files as compressed package transport containers on Windows distribution points - unrelated to email.
- Hazel (macOS automation) rule file - Some versions of Hazel (macOS folder automation tool) used .box for exported rule sets.
Programs that open BOX files
Windows2 apps
Foxmail
Free Open Foxmail and import the .box folder. BOX files are Foxmail's native mailbox format. 
Mozilla Thunderbird (via import)
Open-source Import .box (if mbox-compatible) using the ImportExportTools NG extension. Technical details
deep spec| Format type | Email mailbox container with no single universal specification |
| MIME type | application/octet-stream (no dedicated MIME type registered for .box) |
| Internal layout | Application-dependent: partially binary in Foxmail; text-based mbox-like in Calypso Email |
| Message storage | Sequential RFC 2822 messages (headers + body + attachments) per each application's proprietary record scheme |
| Companion index | Foxmail stores a separate index file alongside each .box folder file |
| Typical file size | 1 MB - several GB depending on mailbox volume |
| Compression | Uncompressed - messages stored as-is at the container level |
| Primary modern user | Foxmail (Windows email client widely used in Chinese-speaking markets, actively maintained) |
| Other known use | Microsoft SMS/SCCM uses .box as binary package transport containers - an entirely unrelated format sharing only the extension |
| Cross-app compatibility | Files from one application are generally unreadable by another due to incompatible internal layouts |
| Associated OS | Windows primarily; Foxmail for Mac extends some support to macOS |
| Import support | Mozilla Thunderbird can import Foxmail .box files via its built-in import wizard |
| Released | 1990s (varied by application) |
| Latest version | N/A (no formal spec; legacy format) |
BOX conversions
Community Q&A
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