Microsoft Outlook uses the Nickfile (NK2) to store e-mail address aliases.
The file type is also known as the Outlook AutoComplete File or the Nickname file.
The actual mime type of the NK2 format is unspecified
The NK2 has the following file signature: hexadecimal: 0D F0 AD BA
Note that other sources claim that the file signature is hexadecimal: 0D F0 AD BA 0A 00 00 00
The NK2 basically contains a list of items. The attributes of these items are defined by the Microsoft Outlook Message API (MAPI).