I am getting all my info from the internet. Below is the site for the macworld article, but it is almost 5 years old. I am using 10.6.8 and i CAN REBUILD MULTIPLE MAIL BOXES FROM THE MENU COMMAND (the article says you can’t) . According to the article below they are the same.
I also just went into the mail folder in users as per the Jim Mitchell article and that works fine. I have only 350 emails and it rebuilt very quickly. I don’t have the personal expertise to say if they are all comparable, but they seem to be.
http://www.macworld.com/article/56673/2007/03/mailfix.html(excerpt from the Article) A possible fix for a slow Mail app
by Rob Griffiths, Macworld.com  Mar 8, 2007 1:00 am
You can do this from within Mail using the Mailbox -> Rebuild command, but you’ll have to do so one mailbox at a time—the menu option is grayed out if you have more than one selected.
