Blocked by preparer's firewall or recipients?

Recipients.

I don't think firewall will block the e-Organizer since the program doesn't generate any internet traffic, AFAIK.

However, the client's antivirus program could either give a warning or block the program outright.  What your client can do is to whitelist the program or ignore the warning.  This is similar to the warning you get when you open Excel file with macro built in.

Since e-Organizer is a niche proprietary program, my hunch (not being an IT expert) is that it is unlikely antivirus software companies will have it whitelisted and there won't be enough users out there to help determine the reputation of the software.