email attachments cannot be opened?

I send email attachments and some people (on PCs) cannot open them - they unzip them ok but they seem to be encrypted??what can I do to sort this ??thanks for any advice

Stefan

You're sending zip files? If that's the case, try transferring them via something other than email, and seeing if they unzip. If they're not zip files, then try sending a variety of files to see if it's a file format problem between your system and theirs.