Convert OST to MBOX — without Outlook

The OST-to-MBOX case is almost always a rescue, not a migration. Someone has an offline cache from an account that no longer exists — a closed tenant, a former employer, a decommissioned server — and Outlook refuses to open it because the account it was bound to is gone. The cached data may still be readable, but only content downloaded before the account became unavailable can be recovered.

Two conversions happen here. First the OST's property sets are resolved into standards-compliant MIME messages, the same reconstruction a PST needs, because neither format stores mail as RFC 5322 to begin with. Then those messages are framed into an mboxrd file — From_ separators, with body lines starting with "From " escaped as ">From " so the mailbox does not split in the wrong place on import.

The escaping is applied to already-escaped runs as well: a body line reading ">From " becomes ">>From ". That is what makes mboxrd reversible — an importer strips exactly one ">" from any line matching ">*From " and recovers the original text byte for byte, which plain mboxo cannot do.

Each message is introduced by a From_ envelope line built from the sender address and the delivery time, formatted as a UTC date string. Two fallbacks are worth knowing before you import: a message with no recoverable sender address gets the placeholder "mailward@localhost", and one with no delivery timestamp gets the Unix epoch — so a handful of messages dated "1 Jan 1970" in Thunderbird means those items carried no usable date property in the cache, not that the conversion failed. The envelope line is metadata for the mbox container; the real Date: and From: headers inside each message are untouched.

Output is one .mbox file per folder, so an OST's folder tree arrives as a set of flat mailboxes rather than a nested hierarchy — you re-create the nesting in the destination client. Outlook-specific properties with no MIME equivalent (categories, flags, some calendar and contact fields) do not survive this or any other PST/OST-to-open-format conversion.

That last caveat deserves precision, because an OST is never just mail. Inside the file, every item — a message, an appointment, a contact, a task, a sticky note — is stored the same way and distinguished only by its message-class property: IPM.Note is mail, IPM.Appointment and the IPM.Schedule.Meeting.* classes are calendar items, IPM.Contact and IPM.Task are what they sound like. The converter does not filter by class: pick your Calendar folder and it converts, with each appointment arriving as a message-shaped record. What survives is the item's text — its subject line and whatever free-text notes it carries. What does not survive is the structured data, because start and end times, recurrence rules, attendee lists, and a contact's phone numbers live in typed MAPI properties that have no MIME header to map onto.

So treat MBOX as the rescue format for mail and for the searchable text of everything else. If the calendar itself is what you need — events you can re-import with their times and recurrences intact — the honest target is iCalendar, and no OST-to-MBOX conversion, ours included, can produce that.

The resulting MBOX can be imported into Thunderbird with ImportExportTools NG or into Apple Mail. Gmail requires an additional step: copy the imported messages from the mail client into the Gmail account over IMAP. Convert only mailboxes you have the right to access.