Rotating Clock Symbol in Mail?

In Leopard mail, the rotating clock symbol or icon is present only when you have 2 or more email accounts showing under the "Inbox." I had two email accounts--one labelled with my email address and the other labelled "On My Mac" [though I never knew what that was] until one day I accidentally somehow eliminated the "On My Mac" account. As a result, only the "Inbox" alone showed up in the lefthand column. Through inference from some discussion on the web, I tried setting up a second email account in the Mail program--this time my Yahoo mail account. Voila! My inbox suddenly again had two email accounts (that is, if you clicked on the little triangle box that "opens" the "Inbox" to show its contents of account names) and, as a result, the little rotating clock symbol started showing up again when I was checking my email.

I still can't figure out what the "On My Mac" email account was, but now I don't care because I have my little rotating clock symbol/icon back to tell me my effort to check mail is in operation.

I hope this helps.