Note: Mail Attachments Iconizer has been upgraded to Attachment Tamer. If you have a license for Iconizer, you can download Attachment Tamer as a free update.

This happens randomly on Mac OS X 10.6.3 and 10.6.4 and perhaps other versions of Mac OS X, and it is not caused by Iconizer. There does not seem anything in particular that triggers the behavior, but it at least does not seem to affect the actual content of the message: the attachment is still there.

Workaround: There is no reliable workaround. Try reinserting the attachment, closing and reopening the message, quitting and relaunching Mail. Note that the issue is not related to Iconizer, so removing Iconizer will not help.