Symptom
Recipients of your email see the text "<Attachment missing>" in the body of the mail message, but the attachment is actually present.
Solution
This issue is resolved by updating to Mac OS X 10.1.
For Mac OS X 10.0 to 10.0.4:
Despite this message, the attachment is still present and can still be opened.
The application Mail, included with Mac OS X, uses the Rich Text (HTML/MIME) message format by default. You can change the format of a message to plain text by choosing the Make Plain Text command from the Format menu in Mail. The best way to avoid the erroneous message is to use the Rich Text format when possible.
When you send a plain text mail that includes an attachment, some mail programs do not recognize the encoding of the message, leading them to display "<Attachment missing>" in the body of the message. This lack of recognition occurs because the message is being encoded in the Alternative MIME standard so it can be compatible with the widest range of recipients. This issue does not affect recipients who are using Mail to view mail.