Running Mac OS X 10.4.11, Pages 3.0.1 and Entourage 11.4.0 or Mail 2.1.1 on various networked MacBook computers.
Discovered a computer with 11.2.4 of Entourage but the same OS. It had a similar problem. Installing Stuffit 11 made no difference.
Two computers running Apple Mail, can seamlessly send and receive a Pages document.
Computers using Entourage as a mail MUA can receive Pages documents as attachments from a sender using Apple Mail MUA, but cannot open them.
Computers using Entourage as a mail MUA may or may not be able to send Pages documents as attachments. Sometimes the error is 4363 as mentioned above for Docx, sometimes a dialogue box gives the choice of cancel or Compress, and sometimes an error dialogue says that the Aladdin System Stuffit Engine cannot be found.
As mentioned previously a file previously compressed into a Zip file will Encode and then send.
Interestingly all Entourage applications have the Compose Preference set to Compression = None, and Encode = Forany Computer (AppleDouble)- so Compression is NOT being requested by the user. The alternative choice (singular) is [ Macintosh(Stuffit) ].
I just tested the same using the alternative choice of using Stuffit. It worked (on this computer anyway). This computer does have Stuffit Engine parts installed in the Library/Frameworks folder.
So choosing the alternative to None for compression, may work for you if you are an Entourage user. I suspect you will need certain parts of Stuffit installed.
Interestingly another preference to check addresses also works the opposite to the preference setting in the same pane. Deleting the Preference file and restarting did not alter this.
If Entourage cannot email it's own Docx format to another Macintosh, then the issue would seem to be in it's ability to handle xml file structures. I do not have 2008 to trial against the problem, but those that use Apple Mail have not had any issues since changing over, so that would seem to be the preferred option for the remaining staff.
