In Challenges in Recovering Deleted Email computers expert witness Steve Burgess writes:
There are three main types of email in common usage – Microsoft Outlook (often paired with a Microsoft Exchange Server), text-based email client programs, and web-based email, or webmail.
In Microsoft Outlook, all emails are kept in one large, encrypted, non-text file – the PST, or Personal Folders file. Outlook has additional functions and additional content as well. There is an integrated address book, multiple mailboxes, a calendar, and a scheduler, all of which are contained within the PST file. When one looks into a PST file with a file editor or word processing application, there is little or nothing intelligible to the human eye. The file content looks like nearly random characters.