There is only one file type that uses the .NNT extension:
Eudora Address Book File
Address book created and used by Eudora, an email client; stores contacts, such as names, nicknames, and email addresses.
NNT files, along with .TOC files, are located in the "EUDORA" directory.
NOTE: Eudora is no longer actively developed.
nndbase.nnt - Default name of the address book given by Eudora.
Updated: December 23, 2014