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A number of people who use Apple's "Mail" email client have installed the WideMail plugin to give an alternate message layout with the message list and message body being side-by-side, which is more appealing for wide-screen displays. After updating to 10.6.4, some of these people have found a couple of problems associated with Mail. The first is that the plugin no longer seems to work, and secondly the Mail subject line is blank in the message browser.
A few users have complained about a problem in OS X 10.6 where Mail will display the text "Missing plug-in" in place of embedded images, both on received mail and on sent messages. This may have occurred after installing a software update for Snow Leopard, but seems to be isolated to OS X 10.6 so far.