What is an OLM file?

A file with .olm extension is a Microsoft Outlook file for Mac Operating System. An OLM file stores email messages, journals, calendar data, and other types of application data. These are similar to PST files used by Outlook on Windows Operating System. However, OLM files created by Outlook for Mac can’t be opened in Outlook for Windows. The OLM file format was introduced with Outlook 2011 to store local data of Outlook. A single OLM file can contain multiple mailboxes, organising the data in the form of folders and subfolders. OLM files can be opened/loaded in Microsoft Outlook 365 and Outlook for Mac.

OLM File Format

OLM are binary files with Application/octet-stream MIME type. The technical specifications of OLM file format are not available openly from Microsoft and there are no format details available for developer’s reference. There are some APIs, however, that can read OLM file format and write these to a PST file for using on Microsoft Windows Operating System such as Aspose.Email.