For years I'm trying to find a keyboard shortcut for 'strikethrough' or 'struck through' or however you want to call text with a horizontal line through it in OS X Mail. You'd think it is a popular feature since the strikethrough style has been on text editors since the earliest days of the Mac, well at least for decades I can remember and on the other hand it's easy to implement in HTML email since there's a HTML tag for it, <s>triketrough, just like there is for <b>old and <i>talic.
I did manage to add a shortcut for strikethrough by adding a style to 'favorite styles' via the 'system wide' styles menu.
(You can do this by making a text selection strikethrough and then add this style to the styles menu via, for example, using the right mouse button or CTRL-clicking on a text selection and choosing Fonts, submenu Styles. I named it Struck though (or maybe it was already there, I'm not sure).
Then I added a shortcut for it in System Preferences / Keyboard / Shortcuts
(Select Applications in the bottem of the left colomn, click the + sign below the second column, choose 'All applications' else it won't work in Textedit for some reason, name it EXACTLY after your menu item, for example in my case Struck through — and choose a keyboard shortcut that is not already in use. Watch out: somehow it doesn't warn you any more for used shortcuts. If I recall correctly it did warm me in older versions of OS X.)
This shortcut is working in Textedit but still not in Mail. I really hoped this new version of Mail 5.2 would finally have a fix for this. Is there really no way to use a shortcut for strikethrough in Mail?