If i send a folder via Apple Mail, will users on other platforms be able to open it?
When you attach a folder to a email in Mail, even thought it looks like an individual folder in the email, both in the composed email and then the sent email, nonetheless the contents of the folder when sent, is zipped into a single .zip archive that is base64 encoded for transport. It is the .zip file that's received by others and there should not be a problem with them accessing it regardless of Host OS.

Looking at the actual .emlx file, here is an excerpt showing the attachment as a .zip file that is actually an "untitled folder" selected to be attached.

--Apple-Mail=_E825218C-47BC-4480-9C03-9F3B81A43257
Content-Disposition: attachment;
    filename="untitled folder.zip"
Content-Type: application/zip;
    x-mac-auto-archive=yes;
    name="untitled folder.zip"
Content-Transfer-Encoding: base64
UEsDBAoAAAAAAAuq2kgAAAAAAAAAAAAAAAASABAAVG8gVGVsbCB0aGUgVHJ1dGgvVVgMAGZ+cFdm
fnBX9QEUAFBLAwQUAAgACAAOqtpIAAAAAAAAAAAAAAAAGwAQAFRvIFRlbGwgdGhlIFRydXRoLy5E
U19TdG9yZVVYDABrfnBXa35wV/UBFADtmDsOwjAQRGeNC0s0LindcABuYEXJCbgABVeg99Eh2hGy


When I received the file in a different email client it was shown as an attachment not the folder as shown in Mail under OS X.

When opened in the other email client it contained the files that were in the "untitled folder" that was attached to the email that was sent.