I have noticed that sometimes TM takes a while to populate the history. You can see that a backup is there, but when you try to get to it, the Finder window is blank for a while until TM can track back through all of the links to the original files. I suspect the same could happen in Mail, particularly if you have a LOT of messages. So what I recommend is to open Mail, then TM and then wait a while to see if it will eventually show you the folders/messages in the backups. It shouldn't take hours, but maybe a lot of minutes.