From Wysiwyg Computers:
It saves all emails from Inbox, sent Items & custom folder as text. When there are lots of messages it is difficult to manage backup. The default Microsoft outlook is available but there are few drawbacks with it. The default backup option saves in .pst format & may get corrupt for very large file size. Also, once it gets corrupted all emails are lost! Here is easier solution is to save all emails in text format. You can copy the messages & search for any text or email in messages.