Free OST Converter
Convert to Any Format

OST (Offline Storage Table) is Microsoft Outlook's format for storing a local copy of your mailbox when you use Exchange or certain account types. The file uses the same binary structure as PST and lets Outlook work offline. OST to MBOX conversion takes that offline cache and exports all emails into a single .mbox file — the standard mailbox archive format used by Mozilla Thunderbird, Apple Mail (mbox), and many Unix/Mac email clients.

Our free OST to MBOX converter processes entirely on your device. You drop a .ost file, and the tool validates the header, scans for email-like content, then writes every extracted message into one MBOX file. All processing is private; nothing is uploaded. This is the standard way to move from Outlook/Exchange to Thunderbird without installing Outlook.

Common use cases include migrating from Outlook to Thunderbird, backing up Exchange cache as a portable MBOX archive, and moving mail to Linux or Mac clients that use MBOX. Because the converter is private, you can use it on any operating system. OST files are typically found on Windows where Outlook has been used with Exchange; converting to MBOX makes the mailbox importable in Thunderbird in one step.

For full fidelity and every attachment, Outlook or a dedicated desktop OST tool may be needed. This tool extracts as much as possible using private scanning and is ideal for migration and backup when you don't have Outlook available.