What is the SBD file format?
An .sbd entity is a directory (folder) used by Mozilla Thunderbird - and previously Netscape Mail and the Mozilla Suite - to store mail subfolders. When a user creates a subfolder inside a mail folder, Thunderbird creates a corresponding .sbd directory named after the parent folder: for example, Inbox.sbd holds all subfolders of the Inbox.
Inside each .sbd folder, Thunderbird stores mail data as mbox-format files (plain text, no file extension), alongside .msf index files that cache folder metadata and message summaries. The hierarchy can be arbitrarily deep: sub-subfolders create further .sbd directories nested inside the parent .sbd.
The .sbd directory itself carries no file content and has no magic bytes - it is a plain filesystem folder. All email messages live in the mbox files inside it. Users should not manually move, rename, or delete .sbd folders while Thunderbird is running; doing so can cause subfolders to disappear from the client until the folder structure is restored. If an .msf index becomes corrupted, Thunderbird rebuilds it automatically on the next launch.
Security & safety
RISK: LOWSBD directories are internal Thunderbird mail storage - they contain email messages in mbox format and are not executable. The contained emails may themselves have malicious attachments, but the .sbd structure itself is safe. Do not manually edit or delete .sbd directories while Thunderbird is running - always close Thunderbird first when manipulating the profile folder.
Format details
in a nutshellPrograms that open SBD files
Windows2 apps
SeaMonkey
Open-source SeaMonkey Mail uses the same profile/folder structure; .sbd directories are opened automatically. 
Mozilla Thunderbird
Open-source Thunderbird opens .sbd directories automatically as part of your mail profile - no manual action needed. Copy the profile folder to restore mail on a new PC. 
macOS1 app
Mozilla Thunderbird
Open-source Thunderbird on macOS reads .sbd subdirectories from ~/Library/Thunderbird/Profiles automatically. 
Linux1 app
Mozilla Thunderbird
Open-source Thunderbird on Linux reads .sbd directories from ~/.thunderbird/Profiles/ automatically. Technical details
deep spec| Entity type | Filesystem directory (folder), not a regular file |
| Naming convention | Parent folder name + .sbd suffix (e.g., Inbox.sbd holds subfolders of the Inbox) |
| Contents | mbox mail files (no file extension) and .msf Mail Summary File indexes |
| Mail format | mbox (RFC 4155) - plain text, UTF-8/ASCII; messages separated by From_ envelope lines |
| Index file format | .msf (Mail Summary File) - proprietary binary cache; auto-rebuilt by Thunderbird if corrupt |
| Nesting support | Sub-subfolders create further .sbd directories inside the parent .sbd, to arbitrary depth |
| Magic bytes | None; .sbd is a directory, not a file |
| Typical data volume | KB to GB depending on number and size of stored email messages |
| Origin | Netscape Mail (circa 1994); continued by Mozilla Suite and Thunderbird 1.0 (December 2004) |
| Editing caution | Must not be moved or renamed while Thunderbird is running; causes subfolder loss until restored |
| Released | circa 1994 (Netscape Mail); Thunderbird continuation from 2003 |
| Specification | www.thunderbird.net |
SBD conversions
Community Q&A
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