What is build in apple mail

As Tim mentioned your question doesn't make sense without more info.

If you're referring to the build of the Mail app it's this:

If you're referring to rebuilding mail boxes it done thus this menu:

Since we're not clairvoyant you'll have to tell us more before we can address the problem without guessing.

Sorry, but your question does not make any sense without context.

https://discussions.apple.com/docs/DOC-5931