A mail file extension is related to the NeoPlanet mail client files.
MAIL file extension - NeoPlanet mail client data
What is mail file? How to open mail files?
File type specification:
The mail file extension is associated with the NeoPlanet, a Trident-shell graphical web browser available for Microsoft Windows operating system.
The mail file stores mail client data from NeoPlanet.
MIME types:
application/x-mailfolder
This mail file type entry was marked as obsolete and no longer supported file format.
This type of file is no longer actively used and is most likely obsolete. This is typically the case for system files in old operating systems, file types from long discontinued software, or previous versions of certain file types (like documents, projects etc.) that were replaced in higher versions of their original programs.