Sending Mac Office spreadsheet files cause error to MS Office user trying to open them

When a user of MS Office for Mac tries to send a MS Office Excel user a file ending with the .xls extension, the recipient get the following message when they try to open it:

"The file you are trying to open is in a different format than specified by the file extension.  Verify the file is not corrupted and from a trusted source."

I tried having the user email the same file to me and got the error message, but when I chose "Open the file" it opened fine.  Is there some conflict with MS Office for Mac and regular MS Office users?  thanks if you can be of any help.