Thunderbird vs Outlook: Email Formats, Storage, and Migration

Thunderbird and Outlook: Two Different Philosophies

Mozilla Thunderbird and Microsoft Outlook are the two most widely used desktop email clients in the world, yet they approach email management from fundamentally different angles. Understanding these differences matters when you need to choose between them, migrate from one to the other, or maintain archives that may outlive either application.

Thunderbird is an open-source project maintained by the Mozilla Foundation. It runs on Windows, macOS, and Linux, stores emails in open standard formats, and gives users full control over their data files. There is no subscription fee, no cloud dependency, and no vendor lock-in by design.

Outlook is Microsoft’s flagship communication tool, tightly integrated with the Microsoft 365 ecosystem. It offers calendar, contacts, and task management alongside email. On Windows, the classic desktop version uses proprietary binary formats for local storage. Outlook is available through Microsoft 365 subscriptions or as a standalone purchase.

Both clients support IMAP and POP3 protocols, so they can connect to the same mail servers. The critical difference lies in how they store your messages locally once downloaded.

How Each Client Stores Your Emails

Thunderbird and the MBOX Format

Thunderbird stores downloaded emails in MBOX files. Each folder in your Thunderbird account corresponds to a single MBOX file on disk. The Inbox is one file, Sent Mail is another, and each custom folder you create generates its own MBOX file.

MBOX is a plain-text format where individual emails are concatenated into one file, separated by a “From ” line delimiter. This simplicity makes MBOX files easy to parse, inspect, and process with standard text tools. Thunderbird also creates companion .msf files that serve as indexes, but these are disposable and can be regenerated.

Your Thunderbird profile directory contains the complete mail store. You can back it up by copying the profile folder, and you can move it between operating systems without conversion.

Outlook and the PST/OST Format

Microsoft Outlook on Windows uses PST (Personal Storage Table) files for local data storage and archiving. A PST file is a structured binary database that holds emails, calendar entries, contacts, tasks, and notes in a single container.

When connected to an Exchange or Microsoft 365 server, Outlook also creates OST (Offline Storage Table) files. OST files are essentially local caches of the server mailbox, formatted similarly to PST but tied to the specific account and machine. Unlike PST files, OST files cannot be opened on another computer without conversion.

The PST format uses an internal B-tree structure for efficient lookups, supports password protection, and can hold both Unicode and legacy ANSI content. Modern PST files (Unicode format, introduced in Outlook 2003) support a theoretical maximum size of 50 GB, though Microsoft recommends keeping them under 20 GB for performance reasons.

MBOX vs PST: Format Comparison

Choosing between these formats often depends on your priorities regarding openness, platform support, and feature set. The table below summarizes the key differences.

Feature	MBOX	PST
Specification	Open standard (RFC 4155)	Microsoft proprietary (published specification)
Platform support	Windows, macOS, Linux	Primarily Windows; limited Mac support
File structure	Plain text, concatenated messages	Binary B-tree database
Content types	Emails only	Emails, calendar, contacts, tasks, notes
Size limit	No formal limit (practical limits apply)	50 GB (Unicode PST)
Encryption	None built-in	Optional password protection
Compression	None	Internal compression available
Ease of parsing	High (text-based)	Low (requires specialized libraries)
Used by	Thunderbird, Apple Mail, many Unix tools	Microsoft Outlook

Both formats store email headers, body content, and attachments. The fundamental difference is that MBOX is a simple sequential text format, while PST is a fully indexed binary database designed for random access to large volumes of structured data.

Moving From Thunderbird to Outlook

Migrating from Thunderbird to Outlook is one of the most common email migration paths for professionals transitioning into Microsoft 365 environments. The process requires converting your MBOX files into PST format, which Outlook can import natively.

The general workflow involves three steps. First, locate your Thunderbird profile folder and identify the MBOX files for each mail folder you want to migrate. Second, convert your MBOX files to PST using a reliable conversion tool. Third, import the resulting PST file into Outlook through the File > Open & Export > Import/Export menu.

You can find a detailed walkthrough in our Thunderbird to Outlook migration guide. The guide covers folder structure preservation, handling large archives, and verifying that attachments transferred correctly.

For a quick conversion, our Thunderbird to Outlook use case page provides a streamlined starting point.

Moving From Outlook to Thunderbird

The reverse migration, from Outlook to Thunderbird, requires converting PST files to MBOX format. This path is common among users switching to Linux, moving away from Microsoft subscriptions, or consolidating onto an open-source platform.

Export your mailbox from Outlook as a PST file if you do not already have one, then convert the PST to MBOX. The resulting MBOX files can be placed directly into Thunderbird’s profile directory or imported through the ImportExportTools NG add-on.

Our Outlook to Thunderbird migration guide covers the full process, including how to handle calendar data and contacts that are stored inside the PST but cannot be represented in MBOX format.

Can You Use Both Simultaneously

Yes. Many users run Thunderbird and Outlook side by side, especially during a transition period or when different accounts are managed by different clients. If both clients connect to the same IMAP server, they will each maintain their own local copy of the mailbox without interfering with each other.

The only scenario that creates complications is POP3, where emails are downloaded from the server and deleted. If two clients pull from the same POP3 account, messages may split unpredictably between them. The solution is to configure POP3 to leave copies on the server, or better yet, to switch to IMAP.

For local archives that are not connected to a server, you will need to maintain separate copies in each format. Converting between MBOX and PST on demand is the practical approach when you need to access the same archive in both clients.

Which Format Is Better for Long-Term Archiving

Long-term email archiving raises specific concerns about format durability, tool availability, and data completeness.

MBOX has the advantage of simplicity and openness. Because it is a plain-text format with a published specification, MBOX files will remain readable for decades. Any programming language can parse an MBOX file, and there is no risk of vendor abandonment. For pure email archiving where you only need to store messages and attachments, MBOX is the safer long-term choice.

PST offers the advantage of completeness. If your archive includes calendar events, contacts, tasks, and notes alongside emails, PST keeps everything in a single container. Microsoft has published the PST format specification (MS-PST), which means third-party tools can read and write PST files without depending on Outlook. However, the binary format is considerably more complex to parse than MBOX.

For organizations with compliance requirements, the best practice is to maintain archives in both formats or use EML files for maximum portability. Each EML file contains exactly one message, making it easy to index, search, and verify individual records.

Our guide on email archiving best practices covers format selection, storage strategies, and retention policies in greater detail.

Convert Between Formats

Whether you are migrating from Thunderbird to Outlook, switching from Outlook to Thunderbird, or building a long-term archive, format conversion is straightforward with the right tool.

MailtoPst supports direct conversion between MBOX and PST in both directions. Upload your source file, select the target format, and download the result. Folder structures, attachments, and metadata are preserved throughout the conversion.

You get 100 MB free credit to start, which is enough to test the process with a representative sample of your archive before committing to a full migration.

Convert now