Maybe this will help..it helped us!
CREDIT goes to:
(I changed some of their original text)
Does the following scenario mimick any of your actions?..
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You have just received an Excel spreadsheet (or perhaps you composed it yourself), and you want to send it to your colleagues on your intranet(Exchange server local) or outside your organization.
You attach the document to your message and you're about to send it, when you remember a small change that needed to be made. You double-click on the attachment at the bottom of the message, make the change, then save and close. But when you go to send the message, something's wrong. Your local users seem to have received it, but it hasn't reached people who aren't on Outlook. You may receive an error message telling you the message was undeliverable due to a conversion failure.
Explanation???
This is what can happen:
1. Supervisor sends you a message with "document1.xls" attached, or you create the message yourself, and attach "document1.xls" to it.
2. Then you open "document1.xls" from within the message and make a small change. You then save and close the document, and hit Send.
3. You discover that recipients who aren't using Outlook haven't received the message, and you receive an error message containing the words "conversion to internet format failed".
***How to avoid this:
Avoid editing documents while they are attached to email messages. Always save a document to your Mac in order to make changes, and attach only finished documents to email messages.
Greg