Switch to the x64 version of 2.1.0.63. If already on x64, try converting in chunks (by folder) rather than whole mailbox.
When moving from on-prem Exchange to Office 365, some OSTs become orphaned. Convert them to PST, then import to Office 365.
Unlike Microsoft’s own methods (which require an active Exchange profile or Export to PST from within Outlook), Coolutils can read an OST file directly from disk and extract all mail items: emails, attachments, calendar entries, contacts, tasks, notes, and journals.
The software ensures that the folder hierarchy and metadata (To, From, Date, Subject) remain identical to the original file.
To appreciate the value of a tool like Coolutils, one must first understand the nature of the OST file. Unlike a PST (Personal Storage Table), which is a standalone file that can be moved between computers and opened in Outlook directly, an OST file is inextricably linked to the specific MAPI profile and Exchange mailbox that created it.
