There is only one file type that uses the .MAILTOLOC extension:
Mail Internet Location File
Email shortcut file created by dragging an email address or a "mailto" link to the Mac OS X desktop; saved in the Mac OS X .PLIST XML format and stores the email address and other optional email fields; used as a fast email shortcut for common recipients.
When MAILTOLOC files are opened, they create a new email in the user's default Mac email program with the "To" field filled. Other fields such as "Cc" and "Subject" are automatically filled if they are included in the MAILTOLOC file.
Updated: September 2, 2011