Files with sqi file extension can be found as message base index files from old Squish/Fidonet.
SQI file extension - Squish message base index
What is sqi file? How to open sqi files?
File type specification:
The sqi file extension was used for message base index files in old Squish program.
Squish is both the name of a FidoNet mail tossing application originally designed for DOS and OS/2, and the name of the primary mail storage format in which this application stores FidoNet and other local BBS messages.
