Outlook Express 4.0, which was included in Windows 98 (June 1998) and integrated with Internet Explorer 4, stored messages in *.mbx files (similar to the Mbox format used in Unix-based systems). Outlook Express 5.0, which was included in Windows 98 Second Edition (SE) (June 1999) and integrated with Internet Explorer 5, switched to *.dbx files, with a separate file for each mailbox folder. Outlook Express 5.01 was included in Windows 2000 (February 2000) and integrated with Internet Explorer 5.01.