Give a try IMM to Thunderbird Converter that can smartly import .IMM to Thunderbird with header information. The tool is able to export IMM to Thunderbird with all attributes- To, Cc, Bcc, Sent, Received, Date, Time, Font, Color, Images, Size, read & unread status. It perfectly works on Windows OS- Windows (XP, Vista, 2003, 7, 8, 8.1 etc).