How to keep your old Outlook mail when you stop using Outlook

First, find out where your mail actually lives

Leaving Outlook is easy or hard depending on one thing: whether your mail is on a server or only on your computer. Outlook’s account settings say which kind each account is.

• IMAP, Exchange or Microsoft 365: the messages are on the server, and Outlook keeps a copy. Any other mail program that signs in to the same account sees the same folders. Nothing needs exporting, as long as the account itself stays open.
• POP: the messages were downloaded and, by default, taken off the server. Outlook’s data file on your computer is the only copy, and when Outlook goes, the way into that file goes with it.
• Archives: older mail that Outlook’s AutoArchive or a colleague moved into a separate .pst is on your computer only, whatever kind the account is.

Work accounts have one more catch: when the organisation closes the mailbox, the server copy goes too, so mail you want to keep has to be exported before that happens, and only where the organisation allows it.

Export while Outlook still runs

In classic Outlook for Windows: File, Open & Export, Import/Export, Export to a file, Outlook Data File (.pst). Choose the account’s top folder and tick Include subfolders, so the whole mailbox goes into one file. Contacts and the calendar can go in the same file.

The newer Outlook for Windows has been gaining .pst features in stages. If yours has no export, classic Outlook is still offered beside it and does the job.

Outlook for Mac exports to an .olm file instead, from File, Export. An .olm is a different format from a .pst, and fewer programs read it, which is one more reason to make the open copy described below.

An .ost file in your Outlook folder is not an export. It is the offline cache of a server mailbox, tied to the profile that made it, and Outlook will not open it on its own once the account is gone. Our article on .ost files covers what can still be done with one.

Keep the file as it is

Copy the exported .pst or .olm to two places before opening it in anything: an external drive and a cloud folder, say. A .pst is a database, and a program that opens it for writing can change it. The copy you never open is the one that is still right in ten years.

Note the size and the message count Outlook reported for each folder. They are the simplest way to tell later whether another program read the whole file.

Make an open copy for the years ahead

A .pst can only be read by a program that implements Microsoft’s database format. mbox (one file per folder) and .eml (one file per message) are plain text that almost every mail program, and any text editor, can read. An archive kept in one of them does not depend on any one program.

• On Linux and macOS, readpst from the free libpst tools turns a .pst into mbox folders.
• Thunderbird imports from Outlook on Windows, but only while Outlook itself is still installed, because it reads the mail through Outlook. Once the mail is in Thunderbird, each folder is an mbox file in its profile.
• Several commercial converters go from .pst or .olm to mbox or .eml. Try one on a small folder first.

Whatever does the converting, check a few folders against the counts you noted, open some message bodies rather than only the subject lines, and open a few attachments. Contacts and calendar entries are the items converters most often drop, so export those separately (.vcf and .ics) if you need them.