MS Excel & Corel Quattro Pro

Hello folks,
I had no problem opening a received MS spreadsheet file with Corel (it's older software, WordPerfect Office 9 on this machine, and whilst opening the file, it seems that Corel didn't even have to convert the file to a new format; it simply opened it), but when I edited the information on the spreadsheet and sent it back as a (Corel) Quattro Pro file, the recipient said he could not open it, which I thought to be a bit surprising. Has anyone had any experience with this? Can MS open Corel spreadsheets?
Thanks in advance,
Chas