ISZ files are “zipped ISO images” that one might used to create a WIN10 install image on DVD.  The ISZ extension should be blocked for incoming email.  As highly specialized ISO imaging software is required to launch ISZ or DAA files — most users are safe.  However ADMINs & tech professionals with a lot of security privileges could mishandle accidentally & allow malicious attacks to occur on inside of networks — where there is softer security controls in place for the intranet for example.

https://isc.sans.edu/forums/diary/Compressed+ISO+Files+ISZ/25252/

While researching a user submitted Direct Access Archive file (DAA), I learned about another file format I too had never heard of before: compressed ISO files, or .isz files.  ISZ files are similar to DAA files: insofar they also contain an ISO file, split in chunks that are then compressed. Like DAA, it’s a proprietary format, however, the ISZ specification is available publicly.