"apple-+" does not increase font size in preview pane in mail.app 4.2

i have letterbox http://harnly.net/2009/software/letterbox/letterbox-beta-4-for-snow-leopard/ installed on my machine. before the SL upgrade, i could hit cmd-+ and increase the font size of the message i am viewing. that was super convenient!

i can't do it any more. any ideas anybody on how to bring that back? btw, if i open a separate window for the message, i can use cmd-+ to increase font size. but no more can i use it in the preview window. 😟