Attaching xlsx files to email should be prohibited. If they want to share data, they can export to PDF and attach that. If data entry or editing is required or the recipient wants to programmatically read that data, they need a proper solution for file sharing and collaboration.

You can use spreadsheets for data entry and maybe formatting, but there is no need to send a full spreadsheet outside your org, when you donâ(TM)t want them to edit the data. Simple solution is to print to (or save as) PDF and also enter a password for encryption at that point. Only visible data is included, no formulas that can break (e.g. string functions with locale-dependent characters like date format) and no as trivial to make changes or send to the wrong person, accidentally or otherwise.