Compatibility Issues Between Mac and Windows?

I recently sent a large quantity of jpgs to our building contractor.
From my mac to their office using Windows machines.
I figured zip-ing them might be appropriate due to the quantity of photos.
Apparently there was a problem at their end, but it is not clear to me what, or rather where the problem might actually be originating from.
Thoughts Please.
attaching a screenshot of the alert from the recipient system.

If you’re using Apple’s, make sure you have “Always send Windows-friendly attachments” selected from the menu Edit->Attachments menu/submenu.

Also be advised that jpgs are already compressed (jpg/jpeg is a lossy compression standard). The act of zipping itself may be causing issues, possibly including the fact that compressing a jpg with zip may cause problems with the file (maybe even increasing its size!).

Alternatively, consider sending individual files using multiple emails if 1st step above does not solve problem.


Check to see if any of the file names have characters Windows can’t support.

According to Microsoft, the following characters must not appear in a filename:

  • < (less-than). Reserved for I/O redirection
  • > (greater-than). Reserved for I/O redirection
  • : (colon). Delimiter to go between device names (e.g. drive letters) and paths
  • " (double-quote). Not sure about this. Probably a delimiter for providing command-line options with spaces and other problematic characters.
  • / (forward slash). Delimiter for command-line options
  • \ (backslash). Windows path separator
  • | (pipe). Reserved for I/O redirection
  • ? (question mark). Reserved for use as a wildcard character
  • * (asterisk). Reserved for use as a wildcard character
  • The NULL character (ASCII 0)
  • So-called “control” characters. (ASCII values 1-31)

Note that macOS doesn’t have these restrictions. When using the UNIX APIs, the only prohibited characters are / (forward slash - the directory separator) and the NULL character. Everything else is valid, although you may need to jump through some hoops to properly quote characters that are normally interpreted by the shell.

And with the Mac API, the only prohibited characters are : (colon - the classic Mac directory separator) and the NULL character.

Interestingly, Apple’s file systems dynamically convert between colons and slashes. Create a file using the UNIX API (e.g. from a Terminal session) that has a colon character and then view it in the Finder and you’ll see the colon converted to a slash. And vice versa.

But the upshot of all this is that you may have created a filename with characters that Windows can’t support. macOS (and Linux) won’t have a problem with this, and neither will the Zip file, but Windows might not be able to unzip it.

If you suspect that this is the case, see if you can rename the problematic files and re-zip them. Or see if a more robust Windows Zip tool (like 7-zip or WinZip) might be able to extract the files (maybe giving you a chance to rename them as a part of the extraction).


My first thought was the name too.

If a lot of images and the folder size is large you could use I’ve never had any issues with them.

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Checking “Always send Windows-friendly attachments” in Apple Mail does not enable Outlook to see them, which is the most common problem I have in sending attachments from Apple Mail. This has been a long-time problem; I am now on Monterey, and had it on earlier versions of Apple Mail.